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Author Topic: what does general relativity say about gravitational action/reaction pairing?  (Read 3223 times)

Offline gem

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what does general relativity say about gravitational action/reaction pairing.

for example in newtons physics if you accelerate a cricket ball upwards in the gravitational field it is thought that the mass of the ball plus its acceleration gives a total force that the ball pulls on the earth and the earth pulls back with a equal and opposite force.

In G R gravity is not a attractive force so what does it postulate in this type of scenario ?


 

Offline Soul Surfer

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It has no effect.  Relativity starts with a basis of Newtons laws and only produces deviations from these under extreme conditions and the only deviations refer to needing more energy to accelerate objects nearer to the velocity of light.

It always annoys me when people say relativity proves newton wrong it proves him right and just adds some small adjustments that relate to things that he could not have been aware of.
 

Offline Geezer

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As SS says. It will make no difference. GR may say gravity is not a force, but GR still says the ball and the Earth will accelerate towards, and collide with, each other.

GR did not repeal Newtonian Mechanics. As SS says, it simply provides a different model that handles certain situations better. (At least I think that's what he meant.)
 

Offline gem

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It has no effect.  Relativity starts with a basis of Newtons laws and only produces deviations from these under extreme conditions
It always annoys me when people say relativity proves newton wrong

I will try not to annoy you but i do intend to look at the differences of the theory's and see how they come to similar conclusions for different reasons,to look at the areas that nature can judge their differences.

As SS says. It will make no difference. GR may say gravity is not a force,
GR did not repeal Newtonian Mechanics. As SS says, it simply provides a different model that handles certain situations better. (At least I think that's what he meant.)

Now i could take what you have both replied two ways GR makes no difference to newtons theory of gravity or the ball will fall to earth because of curved space time just the same as postulated by newton.

 

Offline Geezer

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It has no effect.  Relativity starts with a basis of Newtons laws and only produces deviations from these under extreme conditions
It always annoys me when people say relativity proves newton wrong

I will try not to annoy you but i do intend to look at the differences of the theory's and see how they come to similar conclusions for different reasons,to look at the areas that nature can judge their differences.

As SS says. It will make no difference. GR may say gravity is not a force,
GR did not repeal Newtonian Mechanics. As SS says, it simply provides a different model that handles certain situations better. (At least I think that's what he meant.)

Now i could take what you have both replied two ways GR makes no difference to newtons theory of gravity or the ball will fall to earth because of curved space time just the same as postulated by newton.



By no difference, I mean that you can predict the result by either method, and the observed result will match either prediction.
 

Offline gem

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So what are the dynamics in G R when a mass is accelerating up in relation to earth coming to a point where it stops then accelerates back to earth please include the equivalent to the action reaction of both masses in all three states.
 

Offline Geezer

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So what are the dynamics in G R when a mass is accelerating up in relation to earth coming to a point where it stops then accelerates back to earth please include the equivalent to the action reaction of both masses in all three states.

I'm sure you could do that much better than I could  ;D
 

Offline gem

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Its as the mass is accelerating up through space time in G R i am finding difficult to get the same dynamics of action /reaction as i would with newtons gravitational attraction.
And as you told me on a previous post you have spent 20 years getting your head round relativity i really think you could this much better than i ;D

but seriously can anyone cast some light on this question
 

Offline Soul Surfer

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Let me put it in terms that you are probably familiar with.  Just think of something that you could do in everyday life roll a ball up a smooth level slope and watch it stop and roll back down. nothing is happening very differently so space and time are perfectly normal.  the weight of the ball produces a force down the slope which acts on the velocity  slowing it down and eventually accelerating it downwards.  the rigid surface of the sloping board is producing a reaction force that supports most of the weight of the ball allowing the small horizontal resolved force to produce an acceleration if you had the surface vertically there is no net support from the surface and all the force goes into accelerating the ball towards the earth.
 

Offline JP

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Gem, I don't think anyone here has enough GR background to really do the exact calculations for you.  They're exactly right that in most cases where the masses/densities aren't too great, that Newtonian gravity holds  and you get the familiar action/reaction rules.  How do you get Newtonian gravity from general relativity?  I don't know of a simple explanation of the mathematics, so you'll probably have to consult a textbook (or some online lecture notes.

If you want to push things to the limit where Newtonian gravity fails, the simple rule for action/reaction probably doesn't hold because mass interacts with its own space-time curvature as it moves, which I don't believe happens in Newtonian gravity.  But at that point, comparing the two is useless since Newtonian gravity fails to describe such things anyway!
 

Offline Soul Surfer

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You must remember that General relativistic effects that could cause deviation from newtonian gravity only happens under very extreme physical conditions.  At this point there will be no such thing as familiar solid bodies because electronic bonding and even nuclear bonding is all broken down and everything is quarks gluons electrons and photons all doing their own thing in individual "point particle" energy exchange interactions so the normal concept of an extended body just does not have any relevance.

This os often the case when people present what they see as relativistic paradoxes on these pages.
 

Offline gem

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OK all thanks for the replies, i am off to Spain for a few days so may not be posting much.
I will try to read up on the links in the interim you have provided J P as the ones that you have posted previously have been helpfull.

I think i understand how objects fall due to G R but with out  attractive interaction i don't understand how the mass gets the information of the acceleration and direction values of the other mass.
 

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