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Sunshine

Question Regarding Gravity

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A question i pose about the nature of gravity and its effects, as i understand, gravity is not a force as is commonly believed. It seems as such but, in reality, objects are merely falling toward a larger object in a circular motion caused by the well in the fabric of space time created by that larger object, as per Einstein. Having said that, how is it possible that large bodies, say Jupiter for instance, can physically affect smaller objects such as its moon by way of stretching them and such, causing friction within, leading to geologic activity?. This is where my confusion lies, if gravity is not a force, should there not be any physical effects exerted between bodies in proximity to one another?. Shouldn't they merely orbit each other without physically affecting each other?.

 

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My thoughts are that the curved spacetime idea of gravity seems to be at odds with the proposed existence of gravitons, the carriers of gravitational force.

Gravity is also famous for being just about the only force still preventing a workable Grand Unified Theory from being finalised. Physics seems determined to think about gravity as a manifestation of the other forces.

I think... :)

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I see, time and time again I read about gravity not being a “force” if not, then, the term gravitational “pull” makes no sense. As there shouldn’t be any “pull” between objects, therefore, objects should not by way of anything to do with gravity actually exert any physical change to one another. Yet this is clearly displayed as object deform one another and, even closer to home, the moon itself is the driving force behind our oceans tides. The only way this change could occur is if there IS a force acting upon them. What am I missing here?

Edited by Sunshine
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No-one really knows what gravity is. Gravity is modelled in different ways and as such it is useful sometimes to think of gravity as a force and sometimes as the curvature of spacetime.

Newtonian gravity makes much use of the concept of force and is an almost complete description of how us earth dwellers perceive the effects of gravity. All Newton 'knew' was force.

Einstein, using the notion of a free-fall observer (no force other than tidal forces) was able to extend Newtonian theory by accounting for the very few anomalies it contained.  However, Einstein still needed force (tidal forces in fact) to account for the curvature of his spacetime. The curvature of spacetime and the path of free-fall objects are one.

 

Edited by Tiki
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In GR gravity is caused by the curvature of spacetime. It predicts gravitational waves. Gravitons are an unobserved prediction from a yet to be completed quantum theory of gravity.

The force you feel as the pull of gravity is in fact the push of the electromagnetic force between the material in your feet and the floor as it prevents your free fall along a spacetime geodesic. 

(Yes in Newtons theory it is a force but this theory is less accurate than GR)

Regards Andrew 

Edited by andrew s
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Some physicists believe gravity is tied in with dark energy so once we understand what dark energy is we will understand gravity. Or more likely when and if we discover what dark energy is we’ll have a whole new set of mysteries to solve.

Then there is the dark energy vs dark matter argument of course.

Edited by johninderby
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4 minutes ago, johninderby said:

Some physicists believe gravity is tied in with dark energy so once we understand what dark energy is we will understand gravity. Or more likely when and if we discover what dark energy is we’ll have a whole new set of mysteries to solve.

Then there is the dark energy vs dark matter argument of course.

Can you provide a reference to that view? It is more common, but not universal, view that dark energy is just the cosmological constant of GR I.e. a net residual curvature of spacetime.

Regards Andrew 

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I think Andrew has resolved your problem of curvature and perceived force. The tidal 'forces' only appear because parts of the moon encounter resistance from other parts of the moon because they are trying to fall at different rates. (I think?)

Regarding force, I found this analogy entertaining: two (flat earther :icon_mrgreen:) pilots set off simultaneously from the equator 500 miles apart and fly at the same speed and the same altitude, both using navigation systems which keep them on a course scrupulously perpendicular to the equator. They are astonished to find that, about 12000 miles later, they have collided and have to bale out. While awaiting rescue on the north pole they earnestly discuss what mysterious force caused them to converge...

Olly

Edited by ollypenrice
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There was a paper called “Could Dark Energy be a Manifestation of Gravity?“ 

How they are actually related is simply unkown with plenty of different opinions. One theory is that they are opposing forces. 

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49 minutes ago, johninderby said:

There was a paper called “Could Dark Energy be a Manifestation of Gravity?“ 

How they are actually related is simply unkown with plenty of different opinions. One theory is that they are opposing forces. 

Thanks I have found the paper and will look into it more carefully.  Just reading the abstract it seems to rely on rotation of the Universe which is currently ruled out by observation. 

Regards Andrew 

Just checked it took 7 yrs and 4 versions to get on arXiv and has not been referenced  according to Google scholar.  

Edited by andrew s

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7 minutes ago, johninderby said:

So many different theories. 🤔.

Try searching for Dark Gravity (emergent gravity) 

 

I have looked at most of the current theories but until we have observations that can differentiate them my preferred choice is a residual cosmological constant. 

Regards Andrew 

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Yes there seems to be a new theory every day but no real answers. 

I’m off to investigate the theory of the best biscuit for dunking. 😁

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14 minutes ago, johninderby said:

Yes there seems to be a new theory every day but no real answers. 

I’m off to investigate the theory of the best biscuit for dunking. 😁

Ahead of you there John!

My reference materials on the science of dunking, and some practical applications of materials in a Newtonian universe.

David.😀

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44 minutes ago, johninderby said:

I’m off to investigate the theory of the best biscuit for dunking. 😁

Now there is a good research topic.

Regards Andrew 

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Let's for the moment put gravity aside, and consider following example:

You find yourself in empty space, far from effects of gravity of other bodies and you are imparted a spin along axis that goes thru your chest (front to back). You suddenly feel that "something is pulling" quite strongly on your legs and there is similar sensation in your head but to lesser extent. There is no force applied, yet you feel tension throughout your body - "something is stretching" you, and faster you spin, more uncomfortable it becomes.

What is this force that is doing this? There is no force, yet you feel this effect.

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6 hours ago, Sunshine said:

A question i pose about the nature of gravity and its effects, as i understand, gravity is not a force as is commonly believed. It seems as such but, in reality, objects are merely falling toward a larger object in a circular motion caused by the well in the fabric of space time created by that larger object, as per Einstein. Having said that, how is it possible that large bodies, say Jupiter for instance, can physically affect smaller objects such as its moon by way of stretching them and such, causing friction within, leading to geologic activity?. This is where my confusion lies, if gravity is not a force, should there not be any physical effects exerted between bodies in proximity to one another?. Shouldn't they merely orbit each other without physically affecting each other?.

 

I think what may be causing the confusion is the popular way gravity wells are represented graphically. More often or not the gravity well is represented as a sheet of material on which the planets sit within  the attendant deformation (well).  This allows us to easily visualise how the smaller mass would be attracted or fall into the larger well .  Of course space time is not a 2D sheet of material and neither is the resultant deformation forming a gravity well 2D.  It's hard to graphically represent but the gravity well around say Jupiter projects across all of its volume and not simply a plane through a particular axis.  So with this 3D gravity well we can see how every atom in a nearby moon or planet would therefore be subject to a gravity well created by Jupiter - every atom will experience an acceleration as it falls along a personal space-time geodesic as described by Andrew.  If that acceleration is resisted then the interpretation of that resistance or reaction is what we name force. Those internal reactions (resistance to the gravitational acceleration) in turn can create the heating effect from so called tidal forces.  The main point is that every atom in the moon or second body will indeed be in a gravitational well of Jupiter, the well is not 2D as often drawn. 

Jim 

Edited by saac
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33 minutes ago, vlaiv said:

Let's for the moment put gravity aside, and consider following example:

You find yourself in empty space, far from effects of gravity of other bodies and you are imparted a spin along axis that goes thru your chest (front to back). You suddenly feel that "something is pulling" quite strongly on your legs and there is similar sensation in your head but to lesser extent. There is no force applied, yet you feel tension throughout your body - "something is stretching" you, and faster you spin, more uncomfortable it becomes.

What is this force that is doing this? There is no force, yet you feel this effect.

Would this not be centrifugal force? thanks for the great thought experiment.

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34 minutes ago, vlaiv said:

What is this force that is doing this? There is no force, yet you feel this effect.

That'd be the Centripetal "force".

What you feel is your limbs under strain as they are caused to continually change direction rather than continue in a straight line. The "force" is actually internal strain between the molecules in your body. Not an external action.

Very Newtonian!

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7 minutes ago, Sunshine said:

Would this not be centrifugal force? thanks for the great thought experiment.

An engineer would say yes and happy with that knowledge go on to successfully design high bypass turbo jet aero engines that fly thousands of passengers across the globe. A physicist would roll their  eyes at the use of the term centrifugal.   You have opened a can of worms now sunshine :) 

 

Jim 

Edited by saac
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My name is Jim , I'm an engineer and I believe in F = m^w2r :) 

Jim 

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2 minutes ago, saac said:

An engineer would say yes and happy with that knowledge go on to successfully design high bypass turbo jet aero engines that fly thousands of passengers across the globe. A physicist would roll his eyes at the use of the term centrifugal.   You have opened a can of worms now sunshine :) 

 

Jim 

Yeah, I stand by centrifugal, what we’re describing here is the force felt when inside a spinning “centrifuge” no?

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1 minute ago, saac said:

My name is Jim , I'm an engineer and I believe in F = m^w2r :) 

Jim 

Bahh! Engineers.

In my wood spoiling days we liked to point out that engineers work to thousandths of an inch, we liked to get it right! :)

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