Is Gravity Really a Fundamental Force?

[Polyrhythmic]

3d sphere. I mean to say, In atoms the motion of electrons produces what i believe a bubble of matter. when i think of space-time gravity theroy, i can only see planets rolling around a drain, a 2d flat space. Am I way off base, thinking that way about gravity, or is it more of a 3d bubble solar system.

If you are talking about this picture: http://upload.wikimedia.org/wikipedia/commons/archive/2/22/20101217174549!Spacetime_curvature.png" , you should disregard it, that's not the way gravity works.

 

[Debt]

ty That is helpfull.

 

[Drakkith]

Gravity is the first force, I am asking if a second force that acts opposing gravity. I'm sure I'm just rusty.

Are you asking if there is another force other than the 4 fundamental forces?

 

[danR]

GR states that there is no acceleration due to any "Force" when a particle in free fall enters the presence of a mass; particles in free fall simply start following the geodesics of the curved geometry due to whatever mass - energy distribution is causing the curvature. The equation of geodesic deviation is what quantifies "force" or acceleration in the sense that it gives deviations from this kind of free fall due to the presence of tidal gravitaitonal forces and one can add other forces to the equation.

Sorry to go back in this post, but:

This nicely puts what has been a problem for me: we've imbedded motion in a static 4-space geometry. Why should particles 'simply start following' the geodesics...? What does 'following' mean? Is it simply a description of the necessary direction of entropy? But isn't entropy in this case is driven by the influence of a gravitational 'force'? What is 'forcing' the system? I seem to come back to some kind of 'force'.

Let me illustrate: We place two masses in space, one a ball and the other a torus and insure they are stationary with respect to each other. Why do they follow the geodesic toward each other? Why not away? Or better, why should they move at all?

We know they are going to move together, the ball passing through the torus and we wind up with an oscillating system, losing energy in gravity-waves. But what kick-starts the whole thing? What gets the ball rolling?

 

[WannabeNewton]

Sorry to go back in this post, but:

This nicely puts what has been a problem for me: we've imbedded motion in a static 4-space geometry. Why should particles 'simply start following' the geodesics...? What does 'following' mean? Is it simply a description of the necessary direction of entropy? But isn't entropy in this case is driven by the influence of a gravitational 'force'? What is 'forcing' the system? I seem to come back to some kind of 'force'.

Well geodesic are curves of extremal length. One can derive the geodesic equation by applying the principle of stationary action so particles following geodesics are particles following extremal curves. All particles in classical mechanics seem to obey this principle (analogous to the statement that objects in free fall in flat space follow straight lines) as long as they are in free fall and I don't think we actually know WHY its just the way things are.

 

[danR]

Well geodesic are curves of extremal length. One can derive the geodesic equation by applying the principle of stationary action so particles following geodesics are particles following extremal curves. All particles in classical mechanics seem to obey this principle (analogous to the statement that objects in free fall in flat space follow straight lines) as long as they are in free fall and I don't think we actually know WHY its just the way things are.

I have a vague conjecture (I only have conjectures, and they are all vague anyway) as to why, but I'll start a new post on it, and not interrupt this one.

 

[Mitch Rowe]

Hi everyone..
Ive been thinking about "Gravity" for a while now and I would like to know if anyone thinks I am on the right track or not. Thanks to all who can help in advance.

First I have a question:

If one were to create a rotating spaceship in order to create a sort of "Artificial Gravity" to approximate Earths(Or any strength) "Gravity" then how would Matter react? I mean to say something like: If the air were removed from the spaceship would a hammer and a feather fall at the same rate? Would Matter follow all the rules of gravity(From thhe point of view of the Man standing on the inside edge of the spaceship dropping the hammer and feather) in the spaceship the same as on Earth??

Can I say that from the point of view of the Man in the spaceship that "Gravity" and "Centrifical Force" are the same??

 

[danR]

Hi everyone..
Ive been thinking about "Gravity" for a while now and I would like to know if anyone thinks I am on the right track or not. Thanks to all who can help in advance.

First I have a question:

If one were to create a rotating spaceship in order to create a sort of "Artificial Gravity" to approximate Earths(Or any strength) "Gravity" then how would Matter react? I mean to say something like: If the air were removed from the spaceship would a hammer and a feather fall at the same rate? Would Matter follow all the rules of gravity(From thhe point of view of the Man standing on the inside edge of the spaceship dropping the hammer and feather) in the spaceship the same as on Earth??

Can I say that from the point of view of the Man in the spaceship that "Gravity" and "Centrifical Force" are the same??

Only in the special case of an infinitesimal drop, and selecting the same 'vertical' line from centre to perimeter. As either falls, it will veer from centre and bang obliquely into your instrument panel, since the perimeter is moving much faster than the initial sideways motion of the object.

If they are 'dropped' side-by-side, they will 'fall' at the same rate, but divergently. On Earth they will fall convergently toward the centre of the earth. Hence, only choosing the same vertical line for each. You have to run the experiment separately for each.

So gravity and centrifugal 'force' are rather crappily similar. As long as everything stays put, and is all piled up at the same place, are they the same.

 

[Drakkith]

Let me illustrate: We place two masses in space, one a ball and the other a torus and insure they are stationary with respect to each other. Why do they follow the geodesic toward each other? Why not away? Or better, why should they move at all?

We know they are going to move together, the ball passing through the torus and we wind up with an oscillating system, losing energy in gravity-waves. But what kick-starts the whole thing? What gets the ball rolling?

If my rudimentary understanding of GR is correct, it is because the ball and the torus are moving through spacetime. The curvature of spacetime by mass causes the ball and torus to move closer to each other. There was nothing needed to get it all started because they were already moving.

 

[maverick_starstrider]

If you are talking about this picture: http://upload.wikimedia.org/wikipedia/commons/archive/2/22/20101217174549!Spacetime_curvature.png" , you should disregard it, that's not the way gravity works.

Basically the entire sentiment is awesomely summed up here: http://www.xkcd.com/895/ that comic had me laughing so hard.

 

[Drakkith]

Basically the entire sentiment is awesomely summed up here: http://www.xkcd.com/895/ that comic had me laughing so hard.

Lol, awesome.

 

[Polyrhythmic]

Basically the entire sentiment is awesomely summed up here: http://www.xkcd.com/895/ that comic had me laughing so hard.

True, xkcd is awesome!

 

[danR]

True, xkcd is awesome!

I complained bitterly about the rubber sheet on mySpace 5 years ago. It's a description and a model, and worse, it confuses the beginner by depending on gravity itself to generate the shape of the model. The beginner is in danger of conflating the real gravity needed to generate the model, and the gravitational geometry the model is intended to represent. Even now I look at it and still think: 'yeah, I see, the ball will roll to the centre because gravity is pulling it there, just like real life...'

 

[danR]

If my rudimentary understanding of GR is correct, it is because the ball and the torus are moving through spacetime. The curvature of spacetime by mass causes the ball and torus to move closer to each other. There was nothing needed to get it all started because they were already moving.

Humor me a little further: we have imbedded time and space in a unified Minkowski manifold. There is no movement, only geometry. I suppose as we move a 3-d 'hyperplane' through the object, we can see the two masses start to move toward each other, pass through, generate gravity waves etc. I still need, perhaps, a Planck-scale supermicroscope to see that there is some entropically-driven law, Higg's boson, exchange particles, gauge-fields or whatever, that requires that infinitesimal first step, and that it goes in the 'proper' direction.

I don't like to talk about 'forces', because my university course said they were replaced by field-theory even back in the 19th century, but why do I feel there is still some kind of 'force' needed to kick-start things? Anyway, I started a separate thread to spare this post my vague ramblings.

 

[Polyrhythmic]

I don't like to talk about 'forces', because my university course said they were replaced by field-theory even back in the 19th century, but why do I feel there is still some kind of 'force' needed to kick-start things?

I'd rather say that field theories describe forces, rather than replace them.

 

[danR]

I'd rather say that field theories describe forces, rather than replace them.

Could we say they 'explain' forces? That forces are the surface manifestations of the underlying machinery of fields?

Could gravity be a force after all, in that sense?

 

[Polyrhythmic]

Could we say they 'explain' forces? That forces are the surface manifestations of the underlying machinery of fields?

Could gravity be a force after all, in that sense?

I'd say yes and yes. Where the second question leads us back to what has already been discussed in this thread.

 

[danR]

I'd say yes and yes. Where the second question leads us back to what has already been discussed in this thread.

Sorry, I didn't read the whole thread. Something specific in it caught my attention.

 

[Mitch Rowe]

Only in the special case of an infinitesimal drop, and selecting the same 'vertical' line from centre to perimeter. As either falls, it will veer from centre and bang obliquely into your instrument panel, since the perimeter is moving much faster than the initial sideways motion of the object.

If they are 'dropped' side-by-side, they will 'fall' at the same rate, but divergently. On Earth they will fall convergently toward the centre of the earth. Hence, only choosing the same vertical line for each. You have to run the experiment separately for each.

So gravity and centrifugal 'force' are rather crappily similar. As long as everything stays put, and is all piled up at the same place, are they the same.

OK.. That may be true. But what if the spaceship were as big as a galaxy or an orbit around the Earth? And we only had 10 feet of vertical space to drop the objects. Now we can drop them anywhere and there is no difference.

And I think I have just proved to myself that "Gravity" and "Centrifical Force" are essentially the same.

This is how:
When an Astronaut is in orbit he is essentially weightless. Why? Because he is falling towards Earth but has enough velocity to keep himself from spiraling inwards. If he begins to lose orbit he can increase velocity to maintain it.
So what would happen if another spaceship came by and loaded cargo on to his ship so the ship now has twice as much Mass? Even though to him the extra Mass is essentially weightless shouldn't he have to increase his orbital speed to keep from falling? Actually I guess the spaceship will increase its own velocity due to the fact it has twice as mass for Earth to pull. No.. Thats not right I don't think?. If everything falls at the same rate I guess nothing happens if the spaceship suddenly has twice as much mass other than it begins to fall I suppose. So I suppose the Astronaut must increase speed to counteract Earths gravitational pull?

Would that be right?

 

[Mitch Rowe]

For the first part:
My spaceship in that case is circular and we have 10 feet of space on the inside edge of the ship so that from our point of view we are not moving and can't even see the curvature of the floor!

 

[danR]

OK.. That may be true. But what if the spaceship were as big as a galaxy or an orbit around the Earth? And we only had 10 feet of vertical space to drop the objects. Now we can drop them anywhere and there is no difference.

And I think I have just proved to myself that "Gravity" and "Centrifical Force" are essentially the same.

This is how:
When an Astronaut is in orbit he is essentially weightless. Why? Because he is falling towards Earth but has enough velocity to keep himself from spiraling inwards. If he begins to lose orbit he can increase velocity to maintain it.
So what would happen if another spaceship came by and loaded cargo on to his ship so the ship now has twice as much Mass? Even though to him the extra Mass is essentially weightless shouldn't he have to increase his orbital speed to keep from falling? Actually I guess the spaceship will increase its own velocity due to the fact it has twice as mass for Earth to pull. No.. Thats not right I don't think?. If everything falls at the same rate I guess nothing happens if the spaceship suddenly has twice as much mass other than it begins to fall I suppose. So I suppose the Astronaut must increase speed to counteract Earths gravitational pull?

Would that be right?

For your second part, he has only added mass that already had an identical orbital configuration as his own. This tells me nothing about centrifugal/gravity equivalence.

For your first part, you are almost correct.

I will even agree that if your spaceship had an infinite radius you would be even more correcter.

But centrifugal 'force' is divergent. Your infinite sized spacecraft would have parallel drop direction for any arbitrary non-infinite expanse of floor space (math people might want to object). Gravity is convergent. You will never get that, even in an accelerating elevator.

The only system that could theoretically match gravity would be an expanding balloon-type thing whose surface is accelerating outward in all directions. Then you could drop things at different places and they would fall convergently.

PS. I have a course starting this week, and won't be able to participate much further for a while.

 

[Mitch Rowe]

For your second part, he has only added mass that already had an identical orbital configuration as his own. This tells me nothing about centrifugal/gravity equivalence.

OK but wouldn't the speed of the orbit need to increase to maintain the orbit?? Maybe not? If not then I need to do more homework.

 

[Mitch Rowe]

OK.. You don't have to participate. But I invite anyone to argue with me as I have an idea that I will be getting to if I am not shot down before then.

 

[DaveC426913]

OK but wouldn't the speed of the orbit need to increase to maintain the orbit??

No.

A wrench massing 500g will happily orbit alongside a space shuttle massing 80 tons. As we know everytime we watch a video of things floating inside (or outside) the shuttle.

If you had a 500g wrench and someone came along and added 79,999 more wrenches to it, it would just keep orbiting at the same speed and altitude*.


*Note btw, that the someone came along carrying those 79,999 extra wrenches, had to bring all that momentum with them. They accelerated each and every wrench up to orbital speed before rendezvousing with wrench zero.

 

[danR]

OK.. You don't have to participate. But I invite anyone to argue with me as I have an idea that I will be getting to if I am not shot down before then.

I came back for a minute to see about something I asked (PF is infuriatingly addictive). The trouble is that you are so nearly right (except the astronaut added-mass thing), that you may think you are entirely right.

But that's not good enough for experts. If something is nearly right, they want to see the part that is totally wrong also. I understand there are theories of gravity/inertia/acceleration that demonstrate equivalence under highly restricted circumstances. This entails a mastery of things like manifolds, tensors, gauge-theories, Higgs particles or something likewise esoteric. To understand those theories takes both of us way beyond our abilities to even start to understand.

 

[Mitch Rowe]

I came back for a minute to see about something I asked (PF is infuriatingly addictive). The trouble is that you are so nearly right (except the astronaut added-mass thing), that you may think you are entirely right.

But that's not good enough for experts. If something is nearly right, they want to see the part that is totally wrong also. I understand there are theories of gravity/inertia/acceleration that demonstrate equivalence under highly restricted circumstances. This entails a mastery of things like manifolds, tensors, gauge-theories, Higgs particles or something likewise esoteric. To understand those theories takes both of us way beyond our abilities to even start to understand.

Thats good because I don't want to waste my time on the wrong track. But your idea of an ever expanding space sounds interesting. If space is experiencing something different than Matter.

If the space time continuum warps then it causes an effect of space having a velocity from the point of view of the observer. Maybe that would account for Gravity?

 

[Mitch Rowe]

No.

A wrench massing 500g will happily orbit alongside a space shuttle massing 80 tons. As we know everytime we watch a video of things floating inside (or outside) the shuttle.

If you had a 500g wrench and someone came along and added 79,999 more wrenches to it, it would just keep orbiting at the same speed and altitude*.


*Note btw, that the someone came along carrying those 79,999 extra wrenches, had to bring all that momentum with them. They accelerated each and every wrench up to orbital speed before rendezvousing with wrench zero.

OK Thats right. If an Astronaut climbs out of a spaceship he is still floating and does not fly away or fall to Earth. Hmm?

Thanks.. I am going to have to think about that.

 

[Mitch Rowe]

If the space time continuum warps then it causes an effect of space having a velocity from the point of view of the observer. Maybe that would account for Gravity?

But that wouldn't be right. Then everything would remain weightless. Inertia is just going to have to play a part in Gravity if it is not a force. Wich means Gravity would have to be an effect like Centrifugal Force which is not a force at all. It is the effects of inertia.

So that in order for Gravity to not be a force we must have angular acceleration in either Matter or Space but not both.

 

[bcrowell]

This thread has run its course. There has been a lot of low-quality, speculative discussion, which has generated repeated complaints from users to the mentors. I'm locking it.