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Question Regarding Gravity

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4 hours ago, andrew s said:

If you have two non interacting test particles side by side free falling in a non uniform curved spacetime then they will drift apart. However,  if they are bound, say by covalent bonds, then they will resist this and it is this resistance that is felt as the tidal force.

Regards Andrew 

Yes. This tidal force is the 'real' force of gravity.

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6 minutes ago, Tiki said:

Yes. This tidal force is the 'real' force of gravity.

No in my example it is electromagnetic (em) just as it is the force that stops you free falling through the earth. 

When you jump you free fall back to earth until em stops you.

Maybe, we will have to agree to disagree 😀.

Regards Andrew 

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19 hours ago, andrew s said:

No in my example it is electromagnetic (em) just as it is the force that stops you free falling through the earth. 

When you jump you free fall back to earth until em stops you.

Maybe, we will have to agree to disagree 😀.

Regards Andrew 

Perhaps not😀.

The em force that you refer to is a reactionary force to the gravitational force.

Io's interior heats up because of work done by Jupiter's tidal forces.

Also, if you had a frame of reference in which ALL of the gravitational forces disappeared then then there would be no curvature at all (in that frame the geodesics would remain parallel). In reality, the curvature of spacetime describes the tidal forces, therefore we can justifiably say 'The curvature of spacetime is gravitation'.

Tidal forces are at the heart of GR. Really.

 

 

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On 09/11/2019 at 16:20, vlaiv said:

Not sure about that one. In classical interpretation of gravity hollow sphere (or any other shape) will not have gravitational field inside as gravitational influences of small pieces all cancel each other out perfectly. Have no idea what would be the case in GR though.

Does gravity really cancel out like that? So in this classical interpretation you could float out of the zero gravity interior of the hollowed out planet through a tunnel that the excavators had conveniently left and instantly feel normal gravity when you emerged onto the outer crust? 

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@Tiki all I can do is quote one of the mentors PeterDonis from Physicsforums

In response to Martin Scholtz who said: two observers, although both following geodesics, have non-zero mutual acceleration is coordinate-independent thing and it happens only in the presence of curvature.

PeterDonis replyed: This is tidal acceleration, and since both observers are in free fall, there is no force involved in the GR sense. It's simply spacetime geometry--as you say, tidal acceleration is a sign that spacetime is curved. One could talk of "tidal force", but once again, the force involved is not gravity, but internal forces inside objects that cause stresses due to parts of the object not moving on the geodesic paths that they would move on if the object had no internal forces and every part of it could move independently.

Source https://www.physicsforums.com/threads/is-gravity-a-force.975552/#post-6214766

Regards Andrew

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35 minutes ago, markse68 said:

Does gravity really cancel out like that? So in this classical interpretation you could float out of the zero gravity interior of the hollowed out planet through a tunnel that the excavators had conveniently left and instantly feel normal gravity when you emerged onto the outer crust? 

Yes, a google of "Gravity Inside a hollow sphere " will quickly answer the question in the affirmative. In fact Newton proved the theorem himself.

Regards Andrew

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9 minutes ago, andrew s said:

Yes, a google of "Gravity Inside a hollow sphere " will quickly answer the question in the affirmative. In fact Newton proved the theorem himself.

Regards Andrew

Almost analogous to charge inside a hollow conductor (another inverse square law).

jim  

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2 minutes ago, saac said:

Almost analogous to charge inside a hollow conductor (another inverse square law).

jim  

Exactly, analogous in fact.

Regards Andrew 

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So where are we with this. Are we all in agreement that gravity is not a force but that it can "induce" or cause forces to appear in objects with mass (tidal forces or reaction forces as they may also be considered) . We need a name for this, something to mirror the grandiose of the Copenhagen Interpretation - maybe we could call it The Cloudy Weather Interpretation of Gravity :) 

Jim 

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7 minutes ago, andrew s said:

Exactly, analogous in fact.

Regards Andrew 

Your right Andrew it is I hesitated lol. :)

 

Jim 

Edited by saac
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Has been interesting following this thread but just emphises that we don’t know what gravity is really. 

One of the best quotes I came across was by a physicist who said “Ultimately we don’t know what anything is“

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1 minute ago, johninderby said:

Has been interesting following this thread but just emphises that we don’t know what gravity is really. 

One of the best quotes I came across was by a physicist who said “Ultimately we don’t know what anything is“

I find that a very sad view. We know what gravity is as well as we know anything.  It allowed the prediction of the bending of the path of light by a massive bodies and the gravitational waves that allowed us to observe the merging of neutron stars and black holes.

Yes it may not be the ultimate answer but what more could you reasonably expect?

Regards Andrew 

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16 minutes ago, saac said:

So where are we with this. Are we all in agreement that gravity is not a force but that it can "induce" or cause forces to appear in objects with mass (tidal forces or reaction forces as they may also be considered) . We need a name for this, something to mirror the grandiose of the Copenhagen Interpretation - maybe we could call it The Cloudy Weather Interpretation of Gravity :) 

Jim 

I obviously agree but it is not mass that is the cause of the internal forces but the other forces principally electromagnetic (at normal scales) but also the strong and weak forces at nuclear scale. 

Simply gravity is just geometry and so impacts even zero rest mass particles.

Regards Andrew 

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11 minutes ago, johninderby said:

Has been interesting following this thread but just emphises that we don’t know what gravity is really. 

One of the best quotes I came across was by a physicist who said “Ultimately we don’t know what anything is“

To be fair to john I think this is a matter of language. For example how much do we need to know before we can say "yes we have a pretty good understanding of its nature". Yes there is still much to be known about gravity  - will the force exchange particle (graviton) forever remain theoretical, will we ever be able to describe gravity satisfactorily as a quantum entity.  But as Andrew said we already know way more than the tip of the iceberg and actually the extent of that knowledge and what we have done with it is pretty impressive. Look at the Moon landings, Apollo 13 (gravitational sling shots), satellites, Voyager, Cassini et al.  All  made possible by our knowledge of gravity, how it behaves and its effect on objects with mass. Then there is all the mundane stuff , the bread and butter of engineering: bridges, cars train planes and automobiles :)  More recently, it is surely impossible not to marvel at the team who managed to detect gravitational waves - the precision of that measurement is mind-numbing. So gravity is pretty much well understood I would say but still holds some secrets for us to tame - how exciting is that. :)  

Jim 

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1 hour ago, markse68 said:

Does gravity really cancel out like that? So in this classical interpretation you could float out of the zero gravity interior of the hollowed out planet through a tunnel that the excavators had conveniently left and instantly feel normal gravity when you emerged onto the outer crust? 

No, you would not feel gravity instantly. Imagine you are in a hollow center of a planet and there is a shaft leading to its surface. Inside that central cavity you would not feel any gravitational pull. Once you start climbing up the shaft you would very gradually start feeling gravitational pull. In fact gravity pull that you would experience at certain height inside the shaft would be the same as if there were no mass "above" you - only one "below" you (only mass that is less than your height distance from center of the planet would exert gravitational pull on you - everything that is at larger distance would act as another hollow sphere - thus producing no gravity pull). This leads us to very interesting conclusion - gravity is the strongest on the surface of the planet, once you start going higher away from planet and going down the shaft - it starts reducing.

17 minutes ago, andrew s said:

I find that a very sad view. We know what gravity is as well as we know anything.  It allowed the prediction of the bending of the path of light by a massive bodies and the gravitational waves that allowed us to observe the merging of neutron stars and black holes.

Yes it may not be the ultimate answer but what more could you reasonably expect?

Regards Andrew 

I think this is related to how brain works - or rather how "understanding / knowing" works. In order to know something we need to establish some sort of equivalence - mapping of the phenomena to something that ultimately boils down to our experience.

For example - it is much easier to accept that two electrons repulse one another because most people can relate that there is virtual photon exchange - momentum transfer. At some point this boils down to "hit one billiard ball with another and it will bounce off".

Most people don't have any issues with Newtonian gravity because they relate it to "fields" and them acting as some sort of elastic cord between bodies that pulls them together.

Once you start talking about space time curvature - all analogy goes down the drain. Imagine following: when most people hear about space time curvature - they expect space time to actually be "curved" so space craft orbiting a planet is following this "curve". This means that all points along that trajectory are somehow "curved straight line" or something. But at exact same time and at exact same spatial coordinate - shoot a beam of light, or put object moving at higher velocity. It will no longer follow same path - so there is no actual "curve in space time" (in the sense we started thinking about it) - confusion sets, there is nothing to relate to and we "loose knowledge" - or rather we concluded that we don't know / understand it. :D

 

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47 minutes ago, andrew s said:

I obviously agree but it is not mass that is the cause of the internal forces but the other forces principally electromagnetic (at normal scales) but also the strong and weak forces at nuclear scale. 

Simply gravity is just geometry and so impacts even zero rest mass particles.

Regards Andrew 

Absolutely, I referenced mass so that my object could be influenced by gravity and hence the internal reaction forces would appear. 

But wait, I think I may now have an epiphany due to your explanation Andrew :)   So if we had a sizeable object in a gravity field (deformed space time) even one with no mass would still experience tidal forces as the discrete elements within attempt to follow different geodesic paths, and the em forces between these discrete elements provide the reaction as per Newton's third law; the head would feel the feet being pulled away ! Have I got that right?  I previously thought the object would need mass but now I see that it wouldn't !

Jim 

Edited by saac

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What is interesting is how pathetically weak gravity is.

I can push myself away from the pull of the entire earth!

A tiny magnet can lift a big lump of steel with the whole planet pulling the other way!

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34 minutes ago, Stub Mandrel said:

What is interesting is how pathetically weak gravity is.

I can push myself away from the pull of the entire earth!

A tiny magnet can lift a big lump of steel with the whole planet pulling the other way!

Absolutely yet its reach is infinite and it is really the only fundamental force that we react with on a daily basis at an appreciable level. The em force being another but I think our recognition of that interaction is bit a more esoteric.  

Jim 

Edited by saac

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2 hours ago, vlaiv said:

I think this is related to how brain works - or rather how "understanding / knowing" works. In order to know something we need to establish some sort of equivalence - mapping of the phenomena to something that ultimately boils down to our experience.

For example - it is much easier to accept that two electrons repulse one another because most people can relate that there is virtual photon exchange - momentum transfer. At some point this boils down to "hit one billiard ball with another and it will bounce off".

Most people don't have any issues with Newtonian gravity because they relate it to "fields" and them acting as some sort of elastic cord between bodies that pulls them together.

Once you start talking about space time curvature - all analogy goes down the drain. Imagine following: when most people hear about space time curvature - they expect space time to actually be "curved" so space craft orbiting a planet is following this "curve". This means that all points along that trajectory are somehow "curved straight line" or something. But at exact same time and at exact same spatial coordinate - shoot a beam of light, or put object moving at higher velocity. It will no longer follow same path - so there is no actual "curve in space time" (in the sense we started thinking about it) - confusion sets, there is nothing to relate to and we "loose knowledge" - or rather we concluded that we don't know / understand it. :D

 

I agree with Vlaiv but I think  we also need to acknowledge that we also have different levels of understanding. Ultimately to properly understand reality we would need to turn to a quantum description. However within reality I would say we already have very effective levels of understanding. Effective enough to deliver the technological advances we enjoy and even to deepen our scientific understanding. For example Newtonian physics presents a perfectly adequate and functioning level of understanding of much of our reality, gravity in particular -  It got us to the Moon and back after all. At the same time though we also recognise that there is another deeper level of understanding. I think it is a matter of using the appropriate level of understanding for the appropriate scenario.  So explaining the gravitational lensing we observe of galaxies we replace the Newtonian understanding and invoke GR.   However I wouldn't go worrying about bent space time to launch a satellite into low Earth Orbit, or design the Jame's Web telescope.  

Jim 

 

Edited by saac

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It amuses me (easily done) that it is always mentioned that we don't  have a quantum theory of gravity but never that we don't  have a quantum theory in curved spacetime.

QFT is based on the flat spacetime of SR.

Regards Andrew 

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21 minutes ago, saac said:

 However I wouldn't go worrying about bent space time to launch a satellite into low Earth Orbit, or design the Jame's Web telescope.  

Jim 

 

But, you need to make both GR and SR corrections to make GPS work as the clocks the satellites carry need correcting both for both their velocity and hight in the earths gravitational well.

Regards Andrew 

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39 minutes ago, andrew s said:

But, you need to make both GR and SR corrections to make GPS work as the clocks the satellites carry need correcting both for both their velocity and hight in the earths gravitational well.

Regards Andrew 

I agree and that is really my point Andrew.  We use the level of understanding relevant to the task; acknowledging at the same time its limitations .  It's a bit like using a general service screwdriver rather than the precision screwdriver when the basic screwdriver will suffice. We can place the satellite into orbit using Newtonian mechanics (as we have been doing since Sputnik in 1957). So that understanding whilst not complete is nonetheless effective. It ceases to be effective though for the second task concerning GPS.   I remember a similar discussion on a different forum where attempts were being made to revise the Newtonian understanding of lift as generated by an aircraft's wing.  Here a case was being advanced to supplant Bernoulli and the Navier Stokes equations with a quantum description of lift.  Ultimately a quantum description would probably be more complete but as effective?  Of course in the future who knows, maybe the quantum description may ultimately deliver  more efficient wing profiles.  Out of interest I was watching a programme yesterday by Jim Al-Kahlili  on quantum effects within biology.  He presented on some very compelling and interesting arguments that quantum effects are measurable within some very common biological processes: the Robin's navigation sense, our sense of smell, photosynthesis, and even perhaps a visible impact on evolution.  It was a fascinating programme and showed exactly what I'm trying to explain that we have levels (Russian doll) of understanding each with their own utility.  It's still available on BBC i Player. 

The Secrets Of Quantum Physics - Let There Be Life

Jim 

Edited by saac
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44 minutes ago, andrew s said:

It amuses me (easily done) that it is always mentioned that we don't  have a quantum theory of gravity but never that we don't  have a quantum theory in curved spacetime.

QFT is based on the flat spacetime of SR.

Regards Andrew 

I had to read that twice - brilliant :) 

Bring to mind the much loved of journalist and politicians -"we have made a quantum leap".  And yet they appear so pleased :)   

Jim 

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46 minutes ago, andrew s said:

It amuses me (easily done) that it is always mentioned that we don't  have a quantum theory of gravity but never that we don't  have a quantum theory in curved spacetime.

QFT is based on the flat spacetime of SR.

Regards Andrew 

What seems to be the problem with incorporating curved space time with quantum field theory? Too small contribution of gravity to make significant impact within limits of calculations?

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13 minutes ago, vlaiv said:

What seems to be the problem with incorporating curved space time with quantum field theory? Too small contribution of gravity to make significant impact within limits of calculations?

It is the same issue as getting a quantum theory of gravity.  Namely the treatment of time. QT requires a well defined time coordinate but you don't have one in a curved space time.

Regards Andrew 

 

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