[alancalverd]

I'm quite impervious to insult, ES - years of jousting with a mad chemist can toughen the hide!

There is no such thing as Newtonian gravity or Einsteinian gravity, just gravity.  Masses attract each other,  and we have two usefully predictive mathematical models (thanks to Newton and Einstein) of that attraction.

The time delay between the disappearance of the sun and a change in the earth's orbit  makes intuitive sense until I consider the example of a stone orbiting on the end of a string. If I jerk the string, it's obvious that the impulse travels along the string at the speed of sound and jerks the  stone a few milliseconds later. But if I let go of the string it is somehow counterintuitive that the stone will continue to orbit for a few milliseconds before flying off at a tangent. Deeper thought of course tells me that the reason the speed of sound is finite is because the string has density and elasticity, and can thus continue to exert a force on the stone after I have released it. So in the case of gravitation I'm looking for the gravitational analog of the permittivity and permeability of a vacuum, which determine c for electromagnetic radiation. But it would be a false analog because whilst materials reduce c or absorb e.m. radiation, interposing material between two masses always increases the gravitational force on both.   
 
The more I think about gravity, the less I understand it. But I take comfort from the fact that I'm not alone!

[Bored chemist]

I'm quite impervious to insult, ES - years of jousting with a mad chemist can toughen the hide!

Do you mean "years of being loudly wrong"?

[alancalverd]

Ipsi dixit.Quod erat demonstrandum. Sic semper.

[jeffreyH]

Matter tells spacetime how to bend and spacetime tells matter how to propagate through the gravitational field. So where is the propagation of gravity? Gravitational radiation propagates, the field is static.

About the scenario where the sun suddenly disappears. See here https://image.gsfc.nasa.gov/poetry/venus/q89.html

If gravity propagates then each planet will continue orbiting for different periods of time. Also Pluto, for instance, will be attracted to where the sun was 5.5 hours ago. Not ideal for consistent orbits. The propagation will centre on where the sun was 5.5 hours ago and not where it is simultaneously.

We believe that entangled particles can be in consistently different states at vast distances and yet the gravitational field needing no time to update is horrifying. It is like spooky action at every distance.

[jeffreyH]

While you can state F = ma this is not the same as saying F = mg. No force is felt in the second equation.

I feel a 185 lb force on my feet when I stand, which is due to F=mg so I have to disagree.

No, you feel the force because you are not in free fall. The ground is preventing gravity from moving you through the gravitational field. Therefore, the continuous action of gravity now does feel like a force and compresses your body, causing a detectable stress. Now an accelerometer will work, for the same reasons.

[jeffreyH]

When in free fall, an object can be thought of as being in an approximate inertial frame. This frame is being dragged by gravity towards the source of the field. If this dragging is prevented by the electromagnetic forces holding the ground together, then the object can be thought to be in an accelerating frame. With respect to the dragged inertial frame.

[Origin]

you feel the force because you are not in free fall.

I agree and that force can be calculated by F=mg.

[jeffreyH]

you feel the force because you are not in free fall.

I agree and that force can be calculated by F=mg.

But that describes the force of something acting against gravity. You calculate g, which is the acceleration of an inertial frame in free fall. When the frame is no longer falling the force is due to the prevention of free fall. It is a subtle but important distinction. The force is not a direct consequence of the action of gravity.

[Halc]

I agree and that force can be calculated by F=mg.

Agree, but under GR, the force is applied by the ground, preventing inertial motion.  g is 9.8 m/sec² which is an amount of acceleration, so your formula is really just F=ma where a is a the constant acceleration applied by anything being pushed by the properly accelerating ground.

In Newtonian mechanics, the ground is stationary and applies enough force to counter the object sitting on it. The gravity of Earth applies an acceleration a=GM/r² (and g = GM/r² at a very specific r) which when multiplied by the mass m of the object results in a force of F=GMm/r², which must meet an equal and opposite counter-force from the ground in order to remain stationary.

[gem]

Hi all,
So we seem to be deviating a little from the OP, but there are definitely areas where Newton gravity and Einstein gravity disagree, and it would seem this discussion is coming up against those areas.

So one question that arises out of the last couple of posts is;

How does GR explain the acceleration that occurs during free fall without a force being a factor and how this reconciles with Newtons three laws of motion ?
 

[Eternal Student]

So one question that arises out of the last couple of posts is;

How does GR explain the acceleration that occurs during free fall without a force being a factor and how this reconciles with Newtons three laws of motion ?

  A short answer:   In GR there isn't any acceleration during free fall and that's why no force is required to act.  Inertial frames are not defined in the same way as for Newtonian mechanics.  In GR the inertial frames are actually in free fall.  So, for any object that is in free fall it has 0 acceleration in a local inertial frame and hence 0 force was required.

    How does GR reconcile with Newton's 3 laws?
1.    The 1st law (an object remains at rest or in state of uniform......)  is replaced with   "an object moves along a geodesic".

2.    the 2nd law   F=ma    is usually upgraded to a 4-dimensional version using 4-vectors (because GR works better with tensors).  As indicated above, the definition of an inertial frame is altered, which means that the F and a involved could be different to Newtonian expectations.

3.   The 3rd law (about Equal and Opposite forces) - that can stay   (but see above - inertial frames are different so forces aren't always what you expected from Newtonian mechanics).

4.    I know Newton didn't have a 4th law but there are some things that were tacitly assumed in Newtonian mechanics but need to be redefined or upgraded.  For example, ideas of geometry are upgraded to allow for non-Euclidean metrics.

    Yes, but how does it "reconcile" in the sense of agreeing with Newtonian physics?
Newtonian physics can be recovered as the limiting case under a weak and static gravitational field and low speeds.

Late editing:  I've had most of the night to think over this.  It's difficult to point at the "one thing" that causes Newtonian physics to consider gravity as a force while GR does not, it's not really one thing it's many things.  However, the singe most important thing is probably where you try to re-establish the original form of Newton's first law.  Instead of being happy to accept that objects will move along geodesics (through spacetime) in the absence of any force, it is the insistence that they move in straight lines (through space) unless acted on by a force.

[jeffreyH]

Just to muddy the waters, if gravity was acting on a single elementary particle then you could argue that F = mg would act as a force. Since it is not a composite object.

[Eternal Student]

Just to muddy the waters, if gravity was acting on a single elementary particle then you could argue that F = mg would act as a force. Since it is not a composite object.

   Was that a general reply to previous replies?
   It seems like you are suggesting that every elementary particle needs testing (which could be difficult if you were considering quarks and just wanted to isolate one on its own).  Even if you did test each fundamental particle, are you going test all combinations of those elemantary particles next (which could take a while) or try to invoke some argument about linear superposition of gravitational potential or something?
     Proposing that the gravitational pull on a composite particle (e.g. a Carbon atom) is calculated just as a linear sum of the gravitational effects on its components may seem simple but it isn't:  What are you going to do about "binding energy"?  An atom of carbon has less mass than the sum of its parts, if gravity did pull on the carbon atom just like the sum of pulls on its components then the atom should accelerate slightly faster than g.

[jeffreyH]

Just to muddy the waters, if gravity was acting on a single elementary particle then you could argue that F = mg would act as a force. Since it is not a composite object.

   Was that a general reply to previous replies?
   It seems like you are suggesting that every elementary particle needs testing (which could be difficult if you were considering quarks and just wanted to isolate one on its own).

Well obviously not. Since it is absurd.

Even if you did test each fundamental particle, are you going test all combinations of those elemantary particles next (which could take a while) or try to invoke some argument about linear superposition of gravitational potential or something?

Why on earth would I do that?

Proposing that the gravitational pull on a composite particle (e.g. a Carbon atom) is calculated just as a linear sum of the gravitational effects on its components may seem simple but it isn't:  What are you going to do about "binding energy"?  An atom of carbon has less mass than the sum of its parts, if gravity did pull on the carbon atom just like the sum of pulls on its components then the atom should accelerate slightly faster than g.

How do you come to that conclusion when gravity is not properly understood.? Unless you have solved the outstanding issues. Have you? Hang on, I'll just get some popcorn.

[Eternal Student]

Just to muddy the waters, if gravity was acting on a single elementary particle then you could argue that F = mg would act as a force. Since it is not a composite object.

   I think I'm a bit lost on who was replying with what, to whom and what it implied,  sorry.  Not to worry, it happens often and I usually find my way home.   

[jeffreyH]

Just to muddy the waters, if gravity was acting on a single elementary particle then you could argue that F = mg would act as a force. Since it is not a composite object.

   I think I'm a bit lost on who was replying with what, to whom and what it implied,  sorry.  Not to worry, it happens often and I usually find my way home.   

I was just making an observation. It didn't relate to any one specific post.

[jeffreyH]

This is also of relevance to this thread. https://en.m.wikipedia.org/wiki/Paradox_of_radiation_of_charged_particles_in_a_gravitational_field