# The gravity [what causes it?]

stevendaryl
Staff Emeritus
We fully agree on that one; it's not the same issue.

What's not the same issue?

What's not the same issue?
A firing rocket engine is not the same as the ground; it's not in all aspects the same analysis. The correct way to analyse problems is to work up from simple cases to more complex ones.

In addition, in the same off-topic discussion (but very much on-topic here) it is claimed that :
[..] [Einstein's] "gravitational field" is not a valid physical cause of anything. [..]

stevendaryl
Staff Emeritus
A firing rocket engine is not the same as the ground; it's not in all aspects the same analysis. The correct way to analyse problems is to work up from simple cases to more complex ones.

I don't see a big difference. The ground is held up by pressure forces within the Earth. How is that different than being held up by rockets?

A.T.
If the ground was actively pushing up against you, then it would fly upward...
The movement of the ground is frame dependent. In a free falling frame it is flying upward.

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A.T.
A firing rocket engine is not the same as the ground;
If the rocket applies the same upwards force to the platform as the ground would (so it hovers), then there is no relevant difference. Both forces result in measurable 1g upwards proper acceleration of the platform.

I don't see a big difference. The ground is held up by pressure forces within the Earth. How is that different than being held up by rockets?
It's partially the same as you say, and the unnecessary complication for the analysis that you introduced - even blurring the analysis of what causes gravity - is that a rocket engine is itself a cause of action while the ground is not a cause of action.

PS: The point that I tried to make is in fact reflected by your statement: the ground at the surface of the earth is not pushing outwards, instead it is held up by pressure forces within the Earth. That matches my analysis in post #23

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The movement of the ground is frame dependent. In a free falling frame it is flying upward.
Yes, locally we can pretend that the effect is caused by the upward flying of the ground. However, as we know, the surface of the Earth cannot be flying upward as the radius of the Earth is constant.

Drakkith
Staff Emeritus
Yes, locally we can pretend that the effect is caused by the upward flying of the ground. However, as we know, the surface of the Earth cannot be flying upward as the radius of the Earth is constant.

I think the point A.T. is making is that from the frame of a free falling observer, it is the Earth that is moving, not them.

I think the point A.T. is making is that from the frame of a free falling observer, it is the Earth that is moving, not them.
Yes, we certainly all agree on that; the point that I made is that it is unhelpful for this discussion, as it does not explain the cause of gravity. It was even the explanation of gravity that was debunked in the other thread; the Earth is not expanding (as it should, to make it work for falling observers from all sides).

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Drakkith
Staff Emeritus
I'm sorry, Harry, but I can't figure out how anything from post 23 onward is related to the discussion on how gravity works.

I'm sorry, Harry, but I can't figure out how anything from post 23 onward is related to the discussion on how gravity works.
This discussion: "the gravity [what causes it?] "
Drakkith, there was disagreement if gravity is caused by the gravitational field (really "what causes it"), and suggestions were given that gravity is caused by upward acceleration of the ground ("what causes it" as well as "how gravity works").

A.T.
It's partially the same as you say,
In the sense of the Equivalence Principle, they are exactly the same. The key of the EP is that the experimenter cannot look outside, so he cannot tell if his lab is...

a) standing on the Earth's surface
b) hovering just above the Earth's surface, using rockets
c) firing the same rockets in space, far away from the Earth

The role of the rocket thrust is equivalent to the role of the contact force from the ground, and results in all three cases in the same proper acceleration, which the experimenter can measure. Thus these three cases are equivalent for him. That is the Equivalence Principle.

and the unnecessary complication for the analysis that you introduced
You are the one introducing unnecessary complications, by making a physically irrelevant distinction between active and reactive forces.

davenn
[..] The role of the rocket thrust is equivalent to the role of the contact force from the ground [..]
I'll mention this a last time, as repetition becomes tiring: the ground is held up by pressure forces within the Earth. There is thus no upward (and thus outward) flying ground which idea leads the unaware to imagine expanding space or things like that as cause of gravity, as illustrated in thread https://www.physicsforums.com/threads/poincares-space-dilemma.821480/

PS. Of course you know that just as well as I do; it's just a poor way of explaining the cause of gravity, as opposed to explaining the equivalence of effects.

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A.T.
The role of the rocket thrust is equivalent to the role of the contact force from the ground,
the ground is held up by pressure forces within the Earth.
Just like the chest in case b) is held up by pressure forces in the combustion chamber.
There is thus no upward (and thus outward) flying ground
The chest in case b) also isn't flying outward. So the equivalence I state above still holds true.

what causes the particles to come together
On the surface of the Earth, what cause the lines of longitude, parallel at the equator, to come together at the pole?
The curvature of the surface. The surface of the Earth is not a flat Euclidean 2D space.
Similar answer to your question. According to GR (General Relativity), 4D spacetime is not a flat Euclidean 4D space.

mfb
After almost a lifetime's interest in gravity, and some years ago, this forum gave me a steer to a paper being reviewed by an American professor. It was all about the inflow into matter of some kind of fluid spacetime, and was uncannily like the notion I'd had in my student days. The prime objection to the hypothesis was something like: 'I'm not sure where all this spacetime is going.'

Dale
Mentor
2021 Award
as we know, the surface of the Earth cannot be flying upward as the radius of the Earth is constant.
This reasoning is incorrect in curved spacetime. In a flat spacetime it would indeed be correct that the surface of the Earth could not be accelerating (proper acceleration) outwards while retaining a constant radius, but spacetime is curved and so it can indeed accelerate (proper acceleration) outwards while retaining a constant radius.

To understand the importance of curvature, consider two latitude lines on a sphere. For simplicity consider the latitude lines 5° N and 5° S. As you follow those lines around the sphere, they maintain a constant distance from each other. However, the 5° N line is constantly turning (covariant derivative) to the north and the 5° S line is constantly turning (covariant derivative) to the south. So they are turning away from each other but maintaining constant distance. This is impossible on a flat surface, but possible in a curved surface.

In Schwarzschild geometry you can take any two points on the surface of the earth and find that they are accelerating (covariant derivative = proper acceleration) in different directions, and yet, because the spacetime is curved, the distance between them does not change (e.g. as measured by radar).

Yes, locally we can pretend that the effect is caused by the upward flying of the ground.
Locally we can pretend that the effect is caused by the upward flying of the ground in an inertial frame in flat spacetime. Globally the effect is still caused by the upward flying (proper acceleration) of the ground, but we cannot ignore the curvature of spacetime and there are no global inertial frames.

It was even the explanation of gravity that was debunked in the other thread; the Earth is not expanding (as it should, to make it work for falling observers from all sides).
Hmm, so does a debunking of a debunking equal a rebunking or just a bunking?

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Dale
Mentor
2021 Award
After almost a lifetime's interest in gravity, and some years ago, this forum gave me a steer to a paper being reviewed by an American professor. It was all about the inflow into matter of some kind of fluid spacetime, and was uncannily like the notion I'd had in my student days. The prime objection to the hypothesis was something like: 'I'm not sure where all this spacetime is going.'
Actually, the prime objection is that it does not work in general, but it only works in the Schwarzschild and Kerr spacetimes.

I've always felt that the concept of the Universe changing scale* is no harder to accept than learning it was expanding. Both take quite a leap of faith. In such a universe, spacetime - whatever it turns out to be - would have a limitless sink-hole and limitless energy proportional to the mass it was flowing into.

*every part of every nucleon as well as the space between.

Dale
Mentor
2021 Award
I've always felt that the concept of the Universe changing scale* is no harder to accept than learning it was expanding. Both take quite a leap of faith. In such a universe, spacetime - whatever it turns out to be - would have a limitless sink-hole and limitless energy proportional to the mass it was flowing into.

*every part of every nucleon as well as the space between.
PF is not for personal theory development. The published "flowing spacetime" model works only for Schwarzschild and Kerr spacetimes, and specifically does not work for cosmological spacetimes.

To understand the importance of curvature, consider two latitude lines on a sphere. For simplicity consider the latitude lines 5° N and 5° S. As you follow those lines around the sphere, they maintain a constant distance from each other. However, the 5° N line is constantly turning (covariant derivative) to the north and the 5° S line is constantly turning (covariant derivative) to the south. So they are turning away from each other but maintaining constant distance. This is impossible on a flat surface, but possible in a curved surface.
Clear nice example. I agree.

I love this discussions, We do not know but, There a lot the people looking and training to answer that question , and more difficult thing unite it with everything , some people said, we do not going to get that answer until i travel to others solar systems and other people said we need that answer to travel to others solar systems

I have read that gravity is extremely weak compared to the other three forces. I think this is a big mystery too. Also, being infinite in range.

Drakkith
Staff Emeritus