Why do objects rest on space thus causing gravity?

[ILikeAnswers]

So gravity is supposedly caused by objects such as planets, stars, etc. resting on space which curves space. The more massive the object, the larger the bulge it creates, the stronger the gravity.

My question is, why do these objects even rest on space in the first place? It's as if there's a downward pulling force in space itself. There must be something other than gravity pulling objects down because otherwise, orbits won't exist.

I apologize in advance if I just made a really idiotic thread. I'm not a pro at physics, but I'm hoping to learn.

 

[DaveC426913]

You're talking about the "rubber sheet" analogy of gravity. It is only an analogy. An object resting on a rubber sheet just happens to mimic - in two dimensions - a similar geometry to masses in spacetime in 3 dimensions.

Objects do not "rest on space". There is no "downward pulling".

 

[ILikeAnswers]

Then how do orbits work?

 

[DaveC426913]

Well, that's very hard to describe without using the equations. This is why we use analogies like the rubber sheet.

 

[ILikeAnswers]

Is there at least a basic principle or concept?

 

[russ_watters]

Well, yes -- it's the rubber sheet analogy. It works fine, as long as you don't read past it.

 

[ILikeAnswers]

The analogy though uses the gravity on Earth as some sort of downward pull. If there's no downward pull in space and space is 2D, then how do satellites orbit? Isn't an orbit caused when a satellite is caught in the bulge of space? And that satellite's momentum keeps it in orbit rather than it plummeting to the center of the bulge?

 

[Drakkith]

The bending of the rubber sheet is an analogy for the bending of space. The geometry of space is actually changing because of the presence of mass and energy. So, like a ball rolling across the curved surface of the rubber sheet, an object's path through space is altered because of this change in geometry.

 

[A.T.]

Well, yes -- it's the rubber sheet analogy. It works fine, as long as you don't read past it.

It makes perfect sense... if you don't think about it.

 

[A.T.]

The analogy though uses the gravity on Earth as some sort of downward pull. If there's no downward pull in space and space is 2D, then how do satellites orbit? Isn't an orbit caused when a satellite is caught in the bulge of space? And that satellite's momentum keeps it in orbit rather than it plummeting to the center of the bulge?

No. The spatial distortion, represented by the rubber sheet, has only relatively tiny effects for orbiting bodies. For example, the precession of Mercury's orbit is partialy due to that spatial distortion. The orbiting itself (as opposed to moving straight) is mostly due to the distortion of the time dimension, with respect to space:

 

[russ_watters]

The analogy though uses the gravity on Earth as some sort of downward pull.

I've never heard it described that way.

If there's no downward pull in space and space is 2D, then how do satellites orbit? Isn't an orbit caused when a satellite is caught in the bulge of space? And that satellite's momentum keeps it in orbit rather than it plummeting to the center of the bulge?

The curvature of space isn't "downward", it's 3-dimensional, not 2-dimensional. The satellite feels a pull toward the earth.

 

[A.T.]

The curvature of space isn't "downward", it's 3-dimensional, not 2-dimensional.

Right, but the relevant curvature here is that of space-time, which is 4-dimensional.

The satellite feels a pull toward the earth.

The satellite doesn't "feel a pull", it measures zero proper acceleration.

 

[DaveC426913]

I've never heard it described that way.

This is a common problem I have seen people have with the rubber sheet analogy. They see that a heavy mass is pulled downward onto a rubber sheet by gravity, and they ask why the analogy is circular: it uses gravity to explain gravity.

They are taking the analogy literally. They should really just be seeing the geometry, not the causes of the geometry in the analogy.

 

[A.T.]

They should really just be seeing the geometry

Yes, and when you do that, then it becomes obvious that the whole analogy is bad, because spatial geometry alone cannot explain the"attraction" that causes orbits. If you care only about the geometry, and not about up & down, then you can flip the surface upside down, with a hill instead of the dent. But that would create "repulsion" of the rolling balls.

 

[Micheth]

Yes, and when you do that, then it becomes obvious that the whole analogy is bad, because spatial geometry alone cannot explain the"attraction" that causes orbits.

This has really bugged me about gravity=space curvature.
I get that two objects moving with respect to each other would be diverted from their normal paths by mass-induced curved space. I get that totally, it makes complete intuitive sense.
But to consider two objects at complete rest to each other, why would curved space (or even curved spacetime) start them moving towards each other?
If they're not initially moving, then no curvature would seem to start the ball moving so to speak.
(In the sheet analogy, obviously you've got actual downward pulling gravity that gets the ball moving, but that's just the poor analogy at work)

I just don't get it for actual 4-dimensional gravity.

 

[Drakkith]

Because they arent at rest. They are moving through spacetime.

 

[A.T.]

But to consider two objects at complete rest to each other, why would curved space (or even curved spacetime) start them moving towards each other?

Curved space won't. Curved space-time will.

If they're not initially moving,

They advance in space-time, even when at rest in space.

I just don't get it for actual 4-dimensional gravity.

For a vertical fall, you just need one spatial dimension and time, as shown in the videos in post #10. But this doesn't work for whole orbits, as they require two spatial dimensions and time, so 3 dimensions which are then distorted and would need to be embedded in higher than 3D flat space for visualization.

 

[Micheth]

They advance in space-time, even when at rest in space.

The video was helpful.
I guess the difficult part is accepting the idea of "traveling through time" as if it's just something the apple is moving through.
I think of time as just how rapidly or slowly things move with respect to other things (like clocks, photons, etc.)
I.e. without any movement in the universe I couldn't conceive of time, so it seems like there would have to be movement in the first place for time to have any meaning.
But for space-time, time exists separately and conversely causes movement (by bending space)

Although the equations work I'm told :-)

 

[A.T.]

I think of time as just how rapidly or slowly things move with respect to other things (like clocks, photons, etc.)

Things are aging without moving.

But for space-time, time exists separately

The whole point of space-time is that space and time are not separate.

 

[Micheth]

Things are aging without moving.

No, I don't think you'd age if all your molecules, quarks etc. stopped moving.

The whole point of space-time is that space and time are not separate.

Yeah space-time is one entity. What i mean is "time is something separate in the sense of being a dimension", instead of just how one measures the regularity of movment of objects in space.

 

[A.T.]

Yeah space-time is one entity. What i mean is "time is something separate in the sense of being a dimension", instead of just how one measures the regularity of movment of objects in space.

How time is measured in reality, and how it is modeled in a theory are two independent issues.

 

[Drakkith]

No, I don't think you'd age if all your molecules, quarks etc. stopped moving.

Time would pass normally for you. Consider a muon in space. In its own inertial frame of reference it is not moving at all. Yet time still passes for it and it will eventually decay.

 

[Micheth]

Time would pass normally for you. Consider a muon in space. In its own inertial frame of reference it is not moving at all. Yet time still passes for it and it will eventually decay.

I don't think time would pass for you at all. You'd become "timeless" if everything in you and around you stopped moving...

 

[Drakkith]

I don't think time would pass for you at all. You'd become "timeless" if everything in you and around you stopped moving...

As my example demonstrated, that's wrong. Plain and simple.

 

[Micheth]

As my example demonstrated, that's wrong. Plain and simple.

Not so sure about that because don't muons have spin, and being particles aren't they defined as fluctuations in a field, presumably the electromagnetic field?
So, something's going on inside them that allows "time" to have meaning.
If the muon's field fluctuation just stopped and froze, how would it ever decay?

 

[Drakkith]

Not so sure about that because don't muons have spin, and being particles aren't they defined as fluctuations in a field, presumably the electromagnetic field?
So, something's going on inside them that allows "time" to have meaning.
If the muon's field fluctuation just stopped and froze, how would it ever decay?

Oh I see where you're going. Unfortunately if you're going to break the laws of physics you can't make any reliable claims. I can tell you that our understanding of time is not based on any kind of internal motion or fluctuations, so I see no reason to think that time stops if all motion ceases. But since that's an impossibility anyways it doesn't really matter.

 

[Micheth]

Oh I see where you're going. Unfortunately if you're going to break the laws of physics you can't make any reliable claims. I can tell you that our understanding of time is not based on any kind of internal motion or fluctuations, so I see no reason to think that time stops if all motion ceases. But since that's an impossibility anyways it doesn't really matter.

Well I think you're allowed to bring physics to impossible extremes for the purpose of a thought experiment (Einstein's traveling at light speed on a light beam, for example).
By the way I think I should really have been using a word like "change" instead of "motion".
In a world without any observable change in any entity it would be impossible to conclude that any time has passed, i.e. time would cease to have any meaning.
(no muons decaying, nothing happening).
But in contrast in GR, time already exists (as a component of spacetime), and just the presence of mass existing in it warps it and therefore produces change (motion) in other entities.

 

[A.T.]

In a world without any observable change in any entity it ...

But in contrast in GR...

Yes, physics is about our observable world, not some fictitious one.

 

[Rising Eagle]

My question is, why do these objects even rest on space in the first place? It's as if there's a downward pulling force in space itself. There must be something other than gravity pulling objects down because otherwise, orbits won't exist.

Spacetime is a host habitat in which physical fields, particles, photons reside. Any physical entity has the property of a location in spacetime specified by x,y,z,t. In an orbit around a mass such as earth, the path of an orbiting body like the moon is a 2-D geometric object, either ellipse or circle. But in order to visualize spacetime, we need a time axis. So we can use the plane of the orbital path to visualize a 2-D space with time axis perpendicular to it (a 3-D spacetime). Picture a 3-D cartesian axis system where Earth is centered at the origin with x,y as a horizontal plane and the orbital path of the moon is embedded in the plane. The plane is at t=0 and the time axis rises vertically.

The Earth's path in 3-D spacetime, from its own perspective, is having no motion in the plane and rising vertically through time spontaneously, so it sweeps out the time axis. The moon, however spirals upward in an elliptical motion around the Earth (and the time axis itself as well) as it passes through time. The spiral is itself the distortion of the 3-D spacetime from the motionless Earth that has nonzero velocity only along the time dimension. The moon's spiral path is called a geodesic which is thought of as a path through spacetime that an object may travel by momentum only (i.e., no external forces are pushing on it as seen from the object's own perspective). From the object's perspective, it is in weightless space (often called freefall). There are a continuum of geodesics around the Earth so that objects of different mass and orbital velocity will be in freefall at different radii. These geodesic spirals are all collectively the total distortion of spacetime from the perspective of the earth.

Compare this view with Newtonian (classical) space-only-gravity where the object in orbit is pulled by gravity toward the center of the earth, but is moving so fast tangentially to the pull that the physical surface of the Earth below falls away before the pull of gravity can divert the object from its tangential path and steer the object into the Earth's surface. Instead the object is diverted just enough to stay in orbit. Time is irrelevant in this picture, there are no distortions of space or time, and forces on the moon are nonzero.

There is one additional complication. Both the Earth and moon have mass and each distorts spacetime. There is a point on the line between the center of each of these two masses (a lagrange point and center of total mass) where it can be said that both the Earth and moon are each in orbit around and spiral around this point. Here we are ignoring the pull of the sun, galaxy, and other influences and just considering the isolated earth-moon pair. In this case the distortion of spacetime is visualized by a test mass, whose own distortion of spacetime is negligible, that is launched at various velocities and angles through the earth-moon system and its many possible geodesic paths recorded.

 

[Rising Eagle]

Some additional points can help clarify:

Compare this view with Newtonian (classical) space-only-gravity where the object in orbit is pulled by gravity toward the center of the earth, but is moving so fast tangentially to the pull that the physical surface of the Earth below falls away before the pull of gravity can divert the object from its tangential path and steer the object into the Earth's surface. Instead the object is diverted just enough to stay in orbit. Time is irrelevant in this picture, there are no distortions of space or time, and forces on the moon are nonzero.

By direct mathematical analogy, electrostatic physics (coulomb's law) is identical to the physics of Newtonian gravity where mass is replaced by charge. Once again, the same as in Newtonian gravity, there is no distortion of space. In terms of geodesics, anybody that wanders through a region of undistorted space in freefall (i.e., one that is not interacting with any other body) will sweep out a rectilinear path. Geodesics in undistorted space are all straight lines rather than the curved lines seen in the spacetime of GR in the presence of mass. Any curved paths swept out by an object in Newtonian gravity are caused by explicitly and instantaneously active forces on the body causing it to accelerate in some direction and not by curved geodesics. In comparison to the spiral geodesics seen in distorted spacetime, in Newtonian gravity where distortion of space and of time is not possible, there will be no spiral geodesics, only linear ones. To visualize distortion free vs distortion, a 2-D space and time in Newtonian gravity represented with a cartesian axis system can never have spirals and curves for paths of a freely falling body, but 3-D spacetime in the presence of mass will definitely always have spirals and curves.