What Causes Gravity in the Cosmos?

[thecosmos123456]

we all know gravity but i was wondering how is it caused ? like in a nebula what causes the particles to come together and form a star or any object in the cosmos

 

[Joe Ciancimino]

The short answer is simply, no one knows, we have no clue as to how gravity works. The closest description of gravity is Einstein's second theory of relativity call The General Theory of Relativity. With this, it is described as mass causing the warping and moment of space itself. It does very well to make very accurate predictions but the actual mechanics are still being debated today. Some insist that space itself is either a tensile fabric or perhaps a fluid, while others insist it must be a force carrier such as the predicted Graviton in particle physics. How does gravity actually work? We really do not know yet.

 

[thecosmos123456]

The short answer is simply, no one knows, we have no clue as to how gravity works. The closest description of gravity is Einstein's second theory of relativity call The General Theory of Relativity. With this, it is described as mass causing the warping and moment of space itself. It does very well to make very accurate predictions but the actual mechanics are still being debated today. Some insist that space itself is either a tensile fabric or perhaps a fluid, while others insist it must be a force carrier such as the predicted Graviton in particle physics. How does gravity actually work? We really do not know yet.

but what causes gravity ,means what causes the particles to come closer to each other

 

[Joe Ciancimino]

Mass in General Relativity
https://en.wikipedia.org/wiki/Mass_in_general_relativity

 

[Drakkith]

but what causes gravity ,means what causes the particles to come closer to each other

Science an give you an explanation that tells you how something works, but the ultimate cause of something may never be known. You can always ask, "Well, why does X work the way it does", no matter how deep your explanation goes.

 

[Ravilochana das]

how does Earth's gravity work one way to cause things to remain on it's surface or in a sense stick to it and then also cause something to orbit it like the moon. why didn't Earth's gravity pull the moon right to it's surface instead of keeping it in an orbit? or why didn't the moon's orbits get smaller and smaller until the moon was fully drawn by the Earth's gravity to the Earth's surface?

 

[jtbell]

why didn't Earth's gravity pull the moon right to it's surface instead of keeping it in an orbit?

The moon falls (accelerates) toward the Earth at the same rate that an apple would fall if we could hold it stationary at that distance from the Earth and then let it go. The moon doesn't hit the Earth because it's also moving sideways, so it continuously keeps missing the earth. Likewise if you could shoot a cannon ball at the right speed and if there were no air resistance, it would orbit the earth.

 

[mfb]

Our moon probably formed from debris of a massive collision between Earth and another object - some fragments fell down to earth, some reached an orbit. The latter group formed the moon.

 

[ZapperZ]

The short answer is simply, no one knows, we have no clue as to how gravity works. The closest description of gravity is Einstein's second theory of relativity call The General Theory of Relativity. With this, it is described as mass causing the warping and moment of space itself. It does very well to make very accurate predictions but the actual mechanics are still being debated today. Some insist that space itself is either a tensile fabric or perhaps a fluid, while others insist it must be a force carrier such as the predicted Graviton in particle physics. How does gravity actually work? We really do not know yet.

Actually, you need to be very careful with your wordings here. We know very well ".. how gravity works.."! That's how we managed to predict celestial/astronomical events.

And General Relativity describes how mass affects spacetime, not just space.

So we know quite well how it works. What we don't know yet is how gravity can be incorporated as part of a more unified quantum field theory, or if this is even possible.

And other question on how mass can cause gravity falls under the same group of question of "why charge create electric field" or "what is the origin of quantum spins"... etc... etc.

Zz.

 

[Ravilochana das]

The moon falls (accelerates) toward the Earth at the same rate that an apple would fall if we could hold it stationary at that distance from the Earth and then let it go. The moon doesn't hit the Earth because it's also moving sideways, so it continuously keeps missing the earth. Likewise if you could shoot a cannon ball at the right speed and if there were no air resistance, it would orbit the earth.

could it be that the moon is falling through space but the Earth's gravity prevents it from doing so yet it is not strong enough to pull the moon into itself at that distance so gravity then makes it orbit instead?

 

[mfb]

That does not make sense at all. What does "falling through space" and "preventing it from doing so" even mean?

 

[pbuk]

That does not make sense at all. What does "fallingthrough space" and "preventing it from doing so" even mean?

That's a bit harsh, these might not be the words you would choose, but I think it's quite a good description.

 

[Drakkith]

could it be that the moon is falling through space but the Earth's gravity prevents it from doing so yet it is not strong enough to pull the moon into itself at that distance so gravity then makes it orbit instead?

Instead of asking, "Could it be X", where X is your own description, you're better off learning our current understanding of gravity. Once you do that you will be able to look back and attempt to answer your own question.

 

[Ravilochana das]

That does not make sense at all. What does "falling through space" and "preventing it from doing so" even mean?

perhaps i should have said traveling through space or going through space or wandering through space. i don't know, isn't traveling through space almost like the same thing as falling through space, not that it has to fall fast or wander fast? If gravity from a planet wasn't holding a moon in orbit wouldn't it just keep on going and going (until something's gravity catches it); If our planet got blown up and somehow or moon wasn't for example. Then wouldn't our moon travel through space almost like it were falling? if you got in a space suit and someone dropped you off somewhere in space do you think you would just remain in one place? I guess things wouldn't be able to fall in the direction we term down too far since they would be caught up in the rotation of the galaxy; but wouldn't that also have to do with gravity? i am just giving a hypothetical description about the behavior the moon would have without the influence of Earth's gravity, or how gravity and this natural traveling if you will, of the moon counteract each other to create an orbit.

 

[Drakkith]

There's a very simple way of explaining it. An object moves through space in a straight line unless subjected to a force. Gravity, being a force, causes it to move in a curved line instead. If you do the math, you can figure out what the shape of the curved line is. For the Moon, the shape corresponds to an oval, with the Earth at one focus. Since an oval is a closed shape (compared to a parabola or a hyperbola, which aren't) this means that the Moon is in a closed orbit. If the shape of the line were a parabola or hyperbola then the orbit would be open, and the object would pass nearby and then head off into space and never return.

 

[pbuk]

the moon is falling through space

It would be better to say that the moon is obeying Newton's first law: [...] an object that is in motion will not change its velocity unless an external force acts upon it

but the Earth's gravity prevents it from doing so

It would be better to say that the moon is obeying Newton's second law: [...] the net force applied to a body produces a proportional acceleration

 

[pbuk]

For the Moon, the shape corresponds to an oval, with the Earth at one focus.

No, its an ellipse with the Earth at one focus. An oval is a different shape (that is not followed by any object in freefall).

 

[Drakkith]

No, its an ellipse with the Earth at one focus. An oval is a different shape (that is not followed by any object in freefall).

Ellipse! That's the word I couldn't remember!

 

[rootone]

we all know gravity but i was wondering how is it caused ? like in a nebula what causes the particles to come together and form a star or any object in the cosmos

We don't know how it happens, but it definitely happens. no doubt about that.
We can measure gravity and have clever theories predicting what it will do, and that has proven very useful for things like accelerating space probes with a minimum amount of propellant.
WHY is there gravity?, Well you may a well ask why not?, but it's there.

 

[davenn]

Ellipse! That's the word I couldn't remember!

starting to have those "senior moments" Drak ?

I thought Ol' Jim ( see sig.) and I were the only ones with that problem

 

[Drakkith]

starting to have those "senior moments" Drak ?

I thought Ol' Jim ( see sig.) and I were the only ones with that problem

Tell me about it...
I started work on a crossword that is "easy as pie", according to the title. I'm stumped on about half the ones on the first puzzle.
Words and I don't mix well.

 

[Ravilochana das]

It would be better to say that the moon is obeying Newton's first law: [...] an object that is in motion will not change its velocity unless an external force acts upon it

It would be better to say that the moon is obeying Newton's second law: [...] the net force applied to a body produces a proportional acceleration

i see

 

[harrylin]

The short answer is simply, no one knows, we have no clue as to how gravity works. The closest description of gravity is Einstein's second theory of relativity call The General Theory of Relativity. With this, it is described as mass causing the warping and moment of space itself. [..].

I agree with that except for a few glitches as mentioned by ZapperZ. However, in a parallel thread (off topic there), there is disagreement about how gravitation works.
While I find that it is easy by examination of cause and effect to establish that the action of gravitation is that a body pushes down against the ground, and the ground is passively countering its downward push, the following comment was given:

[..] I can just as well say that I'm countering the ground's push on me. Newtons 3rd Law is symmetrical, so this is an arbitrary choice.

[..] the ground is pushing up on you to counteract gravity [..]

I do think that we have more ways of examination than a narrow look at Newton's third law. If the ground was actively pushing up against you, then it would fly upward if you don't stop it. But it doesn't do so. Inversely, without the ground holding you back, you will fall downward.

 

[stevendaryl]

I do think that we have more ways of examination than a narrow look at Newton's third law. If the ground was actively pushing up against you, then it would fly upward if you don't stop it. But it doesn't do so. Inversely, without the ground holding you back, you will fall downward.

The distinction between a force that is "actively pushing" and one that is "passively pushing" is not standard in physics, as far as I know. But if you consider the two situations:

1. You are standing on a platform that is hovering above the Earth's surface, held up by rockets.
2. You are standing on a platform that is accelerating through empty space, propelled by rockets.

In those two cases, there is no distinction in the force the platform exerts on your feet. There is no distinction in the force the rockets exert on the platform. The distinction is in the trajectories, not the nature of the forces.

 

[harrylin]

The distinction between a force that is "actively pushing" and one that is "passively pushing" is not standard in physics, as far as I know. But if you consider the two situations:

1. You are standing on a platform that is hovering above the Earth's surface, held up by rockets.
2. You are standing on a platform that is accelerating through empty space, propelled by rockets.

In those two cases, there is no distinction in the force the platform exerts on your feet. There is no distinction in the force the rockets exert on the platform. The distinction is in the trajectories, not the nature of the forces.

We agree on that one; it's not the same issue and may cause confusion. However, action and reaction is very standard in physics!

 

[stevendaryl]

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

What's not the same issue?

 

[harrylin]

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.

 

[harrylin]

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]

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.

 

[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.

 

[harrylin]

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

 

[harrylin]

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]

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.

 

[harrylin]

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).

 

[Drakkith]

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.

 

[harrylin]

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.

 

[harrylin]

[..] 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.

 

[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.

 

[eltodesukane]

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.

 

[Rob Benham]

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]

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?

 

[Dale]

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.

 

[Rob Benham]

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]

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.

 

[eltodesukane]

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.

 

[Jose Leal]

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

 

[Louis Philippe]

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]

Thread locked for moderation.