[jeffreyH]

There has to be a distinction between the action of the gravitational field external to the source and the source itself. Density variations in the source affect the action upon external objects. I have seen no evidence that density variations in an object affects the action of the field upon said object. If the field was uniform then those density variations have to be irrelevant. Otherwise the feather and hammer experiment on the moon may have been far less mundane.

[rmolnav (OP)]

I have seen no evidence that density variations in an object affects the action of the field upon said object. If the field was uniform then those density variations have to be irrelevant.

That´s right for relatively small objects. But let us consider our moon, in earth´s gravitational field.
It´s somehow in free falling, because only gravity there affects its movement. But due to its initial speed, it´s rotating around earth (more precisely, around earth/moon barycenter). And it´s tide locked to earth, with two opposite bulges as our planet (but, being all its surface solid, relatively much smaller), one of them towards us.
At last phase of that locking, with very low angular spinning speed, tidal friction was tending to null.
Why did it stop in its current position? My "educated" (?) guess was (long ago) that as it must be anisotropic, in its current position density distribution must be such that some mass concentration happens at farther and nearer parts, corresponding to parts were centrifugal and centripetal forces are bigger, respectively.
It probably even passed a little bit that position, and spinned  back afterwards, with a kind of "oscillating" spin stopping.
That was my guess, and later I learnt some facts that seem to agree with it. But I´ll leave it there.
If moon had been fully isotropic, most probably we´d now be seeing a different "semi-moon" ...
 

 

[rmolnav (OP)]

At last phase of that locking, with very low angular spinning speed, tidal friction was tending to null.
Why did it stop in its current position?

I hope nobody is mislead by that: I mean with very little higher angular speed than the final one of 2π radians avery some 29 days.
And when saying "spinned back" I actually mean that it decreased a little its spin, oscillating around current angular speed, less and less every time ...

[Bill S]

A single object like a black hole doesn't have the energy of the whole universe at it's disposal.

Of course not; but would it not be proportional to the energy of the Universe?

[Bill S]

I know I’ve posted this, or similar, in the past, but have not mastered the skill of finding past posts quickly; so please forgive any repetition

Suppose you are in a box and have with you two marbles. 

Release your marbles simultaneously from the top of the box.  They will fall to the bottom.  If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, because gravity operates as though the entire mass of the gravitating object were at its centre; so the marbles will move towards the centre of the planet; thus they will converge as they fall.

I think this doesn’t challenge the equivalence principle, it just shows that it is, to some extent, an analogy, and as such is open to “nit-picking”

[jeffreyH]

Oh no. On the contrary, that is challenging the equivalence principle. Since it is stated that there is no way to tell the difference.

[rmolnav (OP)]

I think this doesn’t challenge the equivalence principle, it just shows that it is, to some extent, an analogy, and as such is open to “nit-picking”

Oh no. On the contrary, that is challenging the equivalence principle. Since it is stated that there is no way to tell the difference.

Thank you. That difference between your opinions is in the "root" of my decision to initiate this thread ...

[Bill S]

Obviously, one would need ridiculously sensitive equipment to test this, but it seems to work, in principle. 

Another test would be to drop your marbles, simultaneously, from different heights.  They should maintain that difference under acceleration, but move further apart under gravity.     

[rmolnav (OP)]

Obviously, one would need ridiculously sensitive equipment to test this, but it seems to work, in principle. 

Thank you.
Another similar case which challenges the equivalence principle:
Also in the box, but with a long bar (very accurately isotropic) in the box, and comparing the two cases when no gravity at all, and with gravity but in free fall.
If we had sufficiently accurate strain devices installed along the bar, we would find that in one of the cases, the bar never stretches. But in the other, the bar stretches slightly, and more or less depending on its orientation in the box …
This later case would be the one with gravity.
I suppose you´ve guessed why. But I´ll leave it there … Kind of a riddle!

[Bill S]

I take your point, but would the bar stretch if it were orientated at right angles to the direction of the gravitational attraction?

Not clear about the "riddle"; possibly because my brain isn't working too well at the moment.  I hope it's only temporary.

[rmolnav (OP)]

I take your point, but would the bar stretch if it were orientated at right angles to the direction of the gravitational attraction?
Not clear about the "riddle"; possibly because my brain isn't working too well at the moment.  I hope it's only temporary.

Thank you. I do hope the same, if that were the case ... I remember your user´s name from many interesting post!
Perhaps I have the advantage of having been ruminating on similar situations in my very long  discussion on tides (and centrifugal forces), where not uniform distribution of gravitational pull per unit of mass is of paramount importance to really understand those phenomena (should I be more politically correct, and add "as far as I can understand"? ... Frankly, I "feel" really sure ...
The bar would be slightly stretched not when you say ... the opposite: the more in line with the actual gravitational pull, the more.
Why? "Lower" bar parts would be slightly closer, to the C.G. of the celestial object causing gravity, than farther ones. But being a solid, all of them have to accelerate the same. The bar would be accelerating an average, what implies that "lower" parts would be exerting an additional pull (though really, really tiny) on "upper" ones ...
As you said, "one would need ridiculously sensitive equipment to test this", but not more sensitive than, e.g., LIGO´s ...
By the way, the bar even would tend to get in line with the field, rather than staying in another orientation ...
And, if the bar were very, very long ... not special equipment would be needed.

[Colin2B]

Oh no. On the contrary, that is challenging the equivalence principle. Since it is stated that there is no way to tell the difference.

Jeff
I don’t view it that way. I read Einstein’s model of the equivalence principle to be based on a uniform gravitational field, the grav field of a planet is not uniform so you can tell the difference.
Instead of the iron bar idea think about dropping a handful of marbles high  above a planet’s surface they would slowly spread out in the direction of the planet (radially) but draw slightly together tangentially as they fall.

[rmolnav (OP)]

Instead of the iron bar idea think about dropping a handful of marbles high  above a planet’s surface they would slowly spread out in the direction of the planet (radially) but draw slightly together tangentially as they fall.

That´s what Bill S already said in #24 ...
And regarding what you say "I read Einstein’s model of the equivalence principle to be based on a uniform gravitational field, the grav field of a planet is not uniform so you can tell the difference" ... I can´t see your point! As far as I can understand, all gravitational fields would not be uniform, if to consider them "uniform" their "g" ought to be the same whatever the distance to the C.G of the celestial object causing the gravity  !!??

[jeffreyH]

Oh no. On the contrary, that is challenging the equivalence principle. Since it is stated that there is no way to tell the difference.

Jeff
I don’t view it that way. I read Einstein’s model of the equivalence principle to be based on a uniform gravitational field, the grav field of a planet is not uniform so you can tell the difference.
Instead of the iron bar idea think about dropping a handful of marbles high  above a planet’s surface they would slowly spread out in the direction of the planet (radially) but draw slightly together tangentially as they fall.

Yes of course that is right. Sorry Bill.

[Bill S]

Thanks for the kind comment.

Why? "Lower" bar parts would be slightly closer, to the C.G. of the celestial object causing gravity, than farther ones.

No problem with that, but what about if the bar were lying perpendicular to the direction to the direction of the gravitational attraction?  Some slight thickening of the bar, perhaps?

[rmolnav (OP)]

No problem with that, but what about if the bar were lying perpendicular to the direction to the direction of the gravitational attraction?  Some slight thickening of the bar, perhaps?

Thank you.
Well ... If we are "infinitely" accurate, you are right. If cylindrical, it would get a kind of very, very slightly elliptical section.
But it would be in a quite unstable equilibrium:  it should be "perfectly" perpendicular to keep so, and with absolute absence of other affecting factors ... Otherwise, it would slowly tend to get in line with gravity pull direction. Wouldn´t it?
And it could reach that orientation after a little bit oscillating movement, smaller and smaller each time ("∞²" accuracy required now !!)
Well, after all it would be something similar to what happened to our Moon long ago, when it got tidal locked ... Though it continues to oscillate a little bit, due to relatively small "discontinuities" in total dynamic factors that affect our Moon.
 

[Bill S]

Molnav, Just for interest, has the original question/point in the OP been addressed?

I always feel it's a shame that interesting threads rarely have any sort of "conclusion".

[rmolnav (OP)]

Just for interest, has the original question/point in the OP been addressed?
I always feel it's a shame that interesting threads rarely have any sort of "conclusion".

You are right: we have mainly been discussing things somehow in relation to the original question, but not always directly ...
I have in mind a kind of conclusive post, but before I was trying to get more readers´s comments about some details kind of "hidden" in the original question, which I mentioned. Please have a look at:
#4 : "… Do you agree? … If not, don´t hesitate and tell me (but step by step, please ...). If you do, NEXT STEP could be another careful analysis of other quote form linked site: …"
#10: "... I could be wrong, and that´s why I proposed a careful analysis, step by step, of the considered roots of the equivalence principle".
#16: "… In short: I consider that analogies and differences, in cases such as the elevator when still on earth surface, and when far from any gravity field but artificially given "g" acceleration, can be explained within Newton´s Mechanics. Also when comparing an object in free fall (in a gravitational field), and an object really with no gravity (very far from any massive celestial object).
And I was trying to make a detailed analysis of them, to justify my stand. To say things such as "he feels weightless …" …"
In any case, we can´t expect a proper "conclusion" ... What supposedly Einstein deduced from the discussed principle seems to have been proven right ... So, I wouldn´t dare say Einstein was wrong ... Perhaps he saw something more I haven´t even been able to imagine so far !!

[Bill S]

Looking through the # Refs you gave, I think I’m a bit out of my depth, so if I can ask any questions that do no more than highlight the naivety of the “hitch-hiker”, I’m happy to do so, and may well learn something along the way.

The following are a couple of examples. 

#4.  “He DOESN´T actually FEEL any "heaviness" globally … He feels internal stresses caused both by the spaceship push (N.´s 2nd Law: chosen 9.8 m/s2 times its mass m), and inertial reaction forces on each part of his body kind of trying to keep their velocity constant (Newton´s 3rd Law: a total of 9.8m in opposite direction)”.

I’m not clear as to how this impacts on the equivalence principle.

“…since the gravitational acceleration with which an object on earth falls to the ground has that exact same value": that is NOT THE HOLE picture. If that object/person were stopped by some obstacle, he would feel internal stresses originated by both the other object push (N.´s 1st and 2nd Law: upward 9.8m), and gravity pull exerted by Earth on each part of its body (universal gravity law: a total of 9.8m downward)”.

Are you saying that the equivalence principle covers the case where a body in free fall is interrupted by some external influence?

I suspect not, but if you are able to make your points clear to someone with my limited knowledge, I think you are well on the way to achieving the clarity you seem to be seeking.

[rmolnav (OP)]

#4.  “He DOESN´T actually FEEL any "heaviness" globally … He feels internal stresses caused both by the spaceship push (N.´s 2nd Law: chosen 9.8 m/s2 times its mass m), and inertial reaction forces on each part of his body kind of trying to keep their velocity constant (Newton´s 3rd Law: a total of 9.8m in opposite direction)”.
I’m not clear as to how this impacts on the equivalence principle.

If we don´t actually feel gravity directly, but only through a kind of interface (those internal stresses caused by those forces), for me it´s not especially meaningful to say that we feel as weightless when in free fall as when still, but far from any gravitational field (equivalence principle) ... Of course it is so, but simply due to the fact that in the first case the hole gravity attraction is "spent" in moving us with "g" acceleration (no internal stresses in our body), and in the second no force is being exerted on us at all …

“…since the gravitational acceleration with which an object on earth falls to the ground has that exact same value": that is NOT THE HOLE picture. If that object/person were stopped by some obstacle, he would feel internal stresses originated by both the other object push (N.´s 1st and 2nd Law: upward 9.8m), and gravity pull exerted by Earth on each part of its body (universal gravity law: a total of 9.8m downward)”.
Are you saying that the equivalence principle covers the case where a body in free fall is interrupted by some external influence?

Not. But please note the complete quote (from linked site) was:
"The rocket engine of that observer's spaceship is firing and produces an acceleration of 9.8 meters (32 feet) per square second. This accelerated observer feels as heavy as we would feel on earth, since the gravitational acceleration with which an object on earth falls to the ground has that exact same value",
and I was just saying that last sentence "This accelerated ... same value" is NOT THE HOLE picture", that he feels equally "as heavy" only because those internal stresses are caused by equal forces in both cases ...
Not sufficient to deduce that gravity and acceleration are the same thing ! (at least to me ...)