# Gravity

## Recommended Posts

If gravity is a distortion in space time, why are we drawn to the planets interior instead of the direction of the distortion? I think understand the concept of the conical distortion pictures, but im still having trouble understanding why. Does the rotational speed of the celestial body causing the distortion play a part? This may have been touched on before so if you could direct me there that would be awesome.

##### Share on other sites

Rotation has nothing to do with the space-time curvature that results in gravity*.  The curvature is caused by the presence of mass. "Mass tells space-time how to curve, and space-time tells mass how to move." -John Wheeler

*Except for causing frame-dragging, which is a very weak effect, hard to detect, and for most practical purposes, can be ignored.

##### Share on other sites
Posted (edited)

A small correction, regardless of GR or Newtonian's gravity, object is attracted toward center-of-mass not to center of planet. It's just close to the center of planet just in the case of human-scale body and the Earth. In other cosmic scale case center-of-mass will be located somewhere else. e.g. binary stars without significantly bigger one star, will have center-of-mass somewhere between them in the empty space.

Edited by Sensei

##### Share on other sites
25 minutes ago, Janus said:

Rotation has nothing to do with the space-time curvature that results in gravity*.  The curvature is caused by the presence of mass. "Mass tells space-time how to curve, and space-time tells mass how to move." -John Wheeler .

A rotating body is more massive than an otherwise identical non-rotating body.

##### Share on other sites
1 hour ago, Terathorn said:

why are we drawn to the planets interior instead of the direction of the distortion?

What makes you think the center of mass is not the center of distortion ?

2 hours ago, Terathorn said:

I think understand the concept of the conical distortion pictures

Are you maybe confused by the pop-sci analog of the 'bowling ball on a rubber sheet' ?

##### Share on other sites
Posted (edited)
On 7/5/2020 at 4:07 PM, MigL said:

What makes you think the center of mass is not the center of distortion ?

Didn't think of it that way... good point!

Are you maybe confused by the pop-sci analog of the 'bowling ball on a rubber sheet' ?

Indeed i am!!

Forgive my ,what are sure to be, many questions. Kinda trying to teach myself this stuff 🤯🤯

Edited by Terathorn

##### Share on other sites
Posted (edited)
15 hours ago, Terathorn said:

Indeed i am!!

The 'bowling ball on a rubber sheet' is a two dimensional  reduction of a 4 dimensional configuration.
It has multiple problems, one of which is that you can observe it from an embedding third dimension.
Space-time has no embedding dimension; both the bowling ball and you, the observer, would need to be intrinsic to the rubber sheet ( i.e. also two dimensional ).

A three dimensional representation would already get rid of some problems, but not all.
Picture a three dimensional grid, where x, y, and z axis divide up the space into cubic elements.
A mass placed in this space would curve the x, y, and z lines such that the elements are moreskewed, and smaller, as you get closer to the mass.
That is 'space' curvature, and one aspect of gravity, but already much harder to visualize than the two dimensional example of the bowling ball/rubber sheet. Actual gravity is four dimensional 'curvature' of space-time, and I can't help you visualize that as it is impossible.

Some problems are just not suited to visualization, but understanding even just the basics of the math goes a long way to clarifying things.

Edited by MigL

##### Share on other sites
14 hours ago, MigL said:

The 'bowling ball on a rubber sheet' is a two dimensional  reduction of a 4 dimensional configuration.
It has multiple problems, one of which is that you can observe it from an embedding third dimension.
Space-time has no embedding dimension; both the bowling ball and you, the observer, would need to be intrinsic to the rubber sheet ( i.e. also two dimensional ).

A three dimensional representation would already get rid of some problems, but not all.
Picture a three dimensional grid, where x, y, and z axis divide up the space into cubic elements.
A mass placed in this space would curve the x, y, and z lines such that the elements are moreskewed, and smaller, as you get closer to the mass.
That is 'space' curvature, and one aspect of gravity, but already much harder to visualize than the two dimensional example of the bowling ball/rubber sheet. Actual gravity is four dimensional 'curvature' of space-time, and I can't help you visualize that as it is impossible.

Some problems are just not suited to visualization, but understanding even just the basics of the math goes a long way to clarifying things.

That actually makes perfect sense!!

##### Share on other sites
21 hours ago, MigL said:

The 'bowling ball on a rubber sheet' is a two dimensional  reduction of a 4 dimensional configuration.
It has multiple problems, one of which is that you can observe it from an embedding third dimension.
[...]

A three dimensional representation would already get rid of some problems, but not all.
Picture a three dimensional grid, where x, y, and z axis divide up the space into cubic elements.
A mass placed in this space would curve the x, y, and z lines such that the elements are moreskewed, and smaller, as you get closer to the mass.
[...]

Brilliant explanation IMO. +1

##### Share on other sites
Posted (edited)
On 7/9/2020 at 12:17 PM, MigL said:

The 'bowling ball on a rubber sheet' is a two dimensional  reduction of a 4 dimensional configuration.
It has multiple problems, one of which is that you can observe it from an embedding third dimension.
Space-time has no embedding dimension; both the bowling ball and you, the observer, would need to be intrinsic to the rubber sheet ( i.e. also two dimensional ).

A three dimensional representation would already get rid of some problems, but not all.
Picture a three dimensional grid, where x, y, and z axis divide up the space into cubic elements.
A mass placed in this space would curve the x, y, and z lines such that the elements are moreskewed, and smaller, as you get closer to the mass.
That is 'space' curvature, and one aspect of gravity, but already much harder to visualize than the two dimensional example of the bowling ball/rubber sheet. Actual gravity is four dimensional 'curvature' of space-time, and I can't help you visualize that as it is impossible.

Some problems are just not suited to visualization, but understanding even just the basics of the math goes a long way to clarifying things.

If the x,y and z lines are replaced with lines of test particles is it possible  to add to your visualization  by predicting how an identical set up and array would play out in time?

And how it would differ from an outcome where there was spatial  distortion but no temporal distortion?

Or are the spatial and the temporal effects so joined at the hip ** that there is no way to distinguish ,in an observational sense one from the other?

**they don't "self interact" like gravitational sources do, do they?

Edited by geordief

##### Share on other sites
Posted (edited)
On 7/9/2020 at 7:17 AM, MigL said:

A mass placed in this space would curve the x, y, and z lines such that the elements are moreskewed, and smaller, as you get closer to the mass.
That is 'space' curvature, and one aspect of gravity, but already much harder to visualize than the two dimensional example of the bowling ball/rubber sheet. Actual gravity is four dimensional 'curvature' of space-time, and I can't help you visualize that as it is impossible.

Isn’t time subject to the same distortion as space?

On 7/10/2020 at 7:16 AM, geordief said:

Picture a three dimensional grid, where x, y, and z axis divide up the space into cubic elements.

Maybe I’m not doing it right in my head? I’m trying the visualization within the confines of a sphere? You didn’t mention sphere, but I think someone did. Visually it seems to force center mass to a cubic shaped dot. The grid/space within the sphere  is distorted by the center mass the gravitational Effect on me would depend on my position within the sphere in relation to the center mass. As I move that effect might change depending on how my relationship to the mass center changes. If for some reason the mass centers position within the sphere changes again the gravitational effect represented  by the grid/space on me changes. Again, I ask, isn’t time subject to the same distortion as space? Or did I put too much into your suggested mental visualization and overthink it?

Note - I wasn’t  careful enough of where I grabbed my quotes. I think MigL was the original “Quote” I only know how to do one at a time and when I went for the second one I grabbed the first one I came to. I’m sorry if it causes any confusion.

And then there is - apparently nobody mentioned a sphere. I went back quickly and all I found was a bowling ball and a rotating body. Apparently the sphere was a figment of my own dementia...

Edited by jajrussel

##### Share on other sites

Question

I'd like to re-quote a general question I asked on an other thread, as it seems to be relevant to this thread:

"Are the equilibrium states governed by gravity?"

Examples

- An equilibrium state between the Sun and planets.

- An equilibrium state between human body and the Earth.

What I'm Familiar with so Far

I am only familiar with stable, unstable, and maybe dynamic equilibrium state, there are maybe more states that I am not aware of.

Reason

The reason I asked the question is because it seems trying to understand the equilibrium states could lead to make an inference of how gravity work if the gravity is indeed responsible for the equilibrium states, I think.

##### Share on other sites
20 hours ago, tylers100 said:

I'd like to re-quote a general question I asked on an other thread, as it seems to be relevant to this thread:

"Are the equilibrium states governed by gravity?"

Some are, such as planets orbiting a star or satellites orbiting a planet.

But in others, gravity is irrelevant (e.g. chemical or thermal equilibrium).

20 hours ago, tylers100 said:

The reason I asked the question is because it seems trying to understand the equilibrium states could lead to make an inference of how gravity work if the gravity is indeed responsible for the equilibrium states, I think.

I think that's the wrong way round. We know how gravity works already. And we know how it can lead to systems being in stable/metastable/unstable states.

##### Share on other sites
5 hours ago, Strange said:

Some are, such as planets orbiting a star or satellites orbiting a planet.

But in others, gravity is irrelevant (e.g. chemical or thermal equilibrium).

I think that's the wrong way round. We know how gravity works already. And we know how it can lead to systems being in stable/metastable/unstable states.

I see. Thank you for answering.

##### Share on other sites
Posted (edited)
On 7/5/2020 at 5:41 PM, Sensei said:

A small correction, regardless of GR or Newtonian's gravity, object is attracted toward center-of-mass not to center of planet.

Newton showed this to be true only for spherically symmetrical objects, in which case the center of the planet is the same as its center of mass. I just wanted to point this out.

For another shape (a barbell or a hoop for instance), an object nearby may well be attracted away from the center of gravity of the object/system. E.g, if it is noon and a rock is dropped from a tower at the equator (a location between Earth and Sun), the rock will accelerate away from the mutual center of gravity of the sun and Earth.  The moon is beyond the point where this is true, so during a solar eclipse, the moon still accelerates towards the sun, not towards Earth. At half-moon, it accelerates in a direction that points at neither Earth, sun, nor the center of gravity of anything. The moon's path through the solar system is always convex. That means the sun always exerts more force on the moon than does Earth, but this is not the case with our dropped rock.

Quote

In other cosmic scale case center-of-mass will be located somewhere else. e.g. binary stars without significantly bigger one star, will have center-of-mass somewhere between them in the empty space.

Center of mass of our solar system is often not within the sun, such as is the case right now.  It doesn't take much mass of the minor star in a binary system to pull the center of gravity of the system outside the larger star.

Edited by Halc

## Create an account

Register a new account