# Question about Gravity and curvature of space time

A.T.
The reason you don't have to give the ball a push is because it already has a pushing force (gravitational force) acting on it due the curvature of space-time (this is what gives it weight). However, your hand is applying an equal and opposite force upward on the ball that prevents it from falling.
That is basically the Newtonian explanation, with "curvature of space-time" randomly thrown in. In General Relativity there is no "gravitational force", that opposes the force of the hand. That's why the ball experiences proper-acceleration upwards, when held in the hand.

Once you remove your hand from under the ball, the ball accelerates downward in accordance with F=ma.
That is again the Newtonian explanation. In General Relativity there is no force acting in free fall. That why the ball experiences zero proper-acceleration when falling.

A.T.
You see a ball in your hand as a still object because you think you are still too, and the earth is a still referential. Now you must realise that Earth is thrown in space at a very high speed
The OP specifically asks for an explanation, based on the reference frame, where an object is initially stationary. Changing the reference frame to one where it already moves is not really addressing the question, but rather avoiding it.

Actually, the action of gravity on objects, acting like a force, is not easy to understand; a curvature of space is one way of seeing it,
General Relativity describes gravity as curvature of space-time, not just space. Even when you change the reference frame to one where the ball already moves, you will not get the right quantitative result considering only spatial curvature.

Nugatory
Mentor
The OP specifically asks for an explanation, based on the reference frame, where an object is initially stationary. Changing the reference frame to one where it already moves is not really addressing the question, but rather avoiding it.
I think you are misunderstanding stefjourdan here - I read his as post as trying to explain that there is no such thing as absolute rest.

A.T.
I read his as post as trying to explain that there is no such thing as absolute rest.
I understand that part. But it doesn't change the fact, that there are frames where the falling object is initially at rest. And that's where the OP wants gravity due to curvature of space-time explained.

Talking about some other frames, where Earth and ball are moving very fast is not addressing this, and gives the wrong impression that the movement of Earth and ball in some arbitrary frame is relevant for their gravitational attraction.

Advising the OP to "forget time" and consider only spatial curvature is similarly misleading. Even in the frames where the object already moves, you still need to consider space-time, not just space curvature.

I think one more piece is required to fully comprehend "why": Gravity is virtually indistinguishable from acceleration through space which nicely removes the time factor for simplified comprehension. Imagine standing in a rocket in empty space on a floor perpendicular to acceleration. If you hold a ball, your hand is accelerating it and if you let go it stops accelerating with you and the floor accelerates to reach it. I hope that helps!
I don't know why I wrote time, I meant gravity...

I understand that part. But it doesn't change the fact, that there are frames where the falling object is initially at rest. And that's where the OP wants gravity due to curvature of space-time explained.

Talking about some other frames, where Earth and ball are moving very fast is not addressing this, and gives the wrong impression that the movement of Earth and ball in some arbitrary frame is relevant for their gravitational attraction.

Advising the OP to "forget time" and consider only spatial curvature is similarly misleading. Even in the frames where the object already moves, you still need to consider space-time, not just space curvature.
Thank you for adding this. I feel vindicated somewhat. The other posters addressed other interesting issues but not really my question directly so I kind of gave up figuring I was out of my league.

stevendaryl
Staff Emeritus
Thank you for adding this. I feel vindicated somewhat. The other posters addressed other interesting issues but not really my question directly so I kind of gave up figuring I was out of my league.
To me, it helps to visualize curved spacetime by thinking about simpler analogies. For example, consider the surface of a globe, where east-west represents spatial separation and north-south represents time. An object just sitting do nothing is still traveling in time--the time coordinate is increasing. On the globe analogy, that means that all objects that are sitting still in space (not traveling east-west) are still traveling north (forward in time). But as two objects travel northward along two lines of longitude, the spatial distance between them decreases (till you get to the north pole, where all lines of longitude come together). So objects traveling in time (northward) get closer together in space (east-west separation) due to the spacetime being curved.

A.T.
Thank you for adding this. I feel vindicated somewhat. The other posters addressed other interesting issues but not really my question directly so I kind of gave up figuring I was out of my league.
In think the opposite is the case. The question you raised, is the typical question of an intelligent person, who is rightly confused by the misleading rubber-sheet analogy. Some of the responses were typical for people who uncritically swallow this analogy, because they are happy with a mere superficial feeling of understanding something. The other extreme are people who see the flaws of the analogy, and wrongly conclude that the theory itself is flawed.

PeterDonis
Mentor
2019 Award
The reason you don't have to give the ball a push is because it already has a pushing force (gravitational force) acting on it due the curvature of space-time (this is what gives it weight). However, your hand is applying an equal and opposite force upward on the ball that prevents it from falling.
The first part of this is not correct; the effect of spacetime curvature by itself does not cause the ball to feel weight. If the ball were moving purely due to the curvature of spacetime (i.e., if your hand were not applying a force to it), it would be weightless. What causes it to feel weight is the force your hand exerts to prevent it from falling.

Some purists (such as myself) would not even call the effect of spacetime curvature a "force", precisely because it does not cause the ball to feel weight; we reserve the term "force" for something that is actually felt as weight (like the force your hand exerts on the ball). But others allow the term "force" to be used to describe the effect of spacetime curvature on the ball, at least in the weak field approximation that applies to this discussion; this allows you to account for the ball being motionless, in a frame in which the ground is at rest, by saying that it has two equal and opposite forces acting on it. But you have to be careful to keep in mind that one of these "forces" is not felt as weight.

Just testing some words here. Is it fair or even accurate to say that in the GR analysis the force of your hand on the ball is not balanced by the ball (specifically contrary to the Newtonian spatial "equilibrium" of the ball's weight and your hand's reaction to it), causing the ball to be accelerated away from it's geodesic? In which case there is no equilibrium and no reaction force on your hand, and the thing you feel as weight is just the inertial resistance of the ball to this acceleration.

Hope that is clear . . .

A.T.
Is it fair or even accurate to say that in the GR analysis the force of your hand on the ball is not balanced by the ball (specifically contrary to the Newtonian spatial "equilibrium" of the ball's weight and your hand's reaction to it), causing the ball to be accelerated away from it's geodesic?
Yes, that's what proper acceleration is: deviation from a geodesic worldline.

In which case there is no equilibrium and no reaction force on your hand,
No, that's wrong. If you apply a force to the ball, the ball applies an equal but opposite force to your hand. Whether your force on the ball is balanced by other forces on the ball is irrelevant for this.

and the thing you feel as weight is just the inertial resistance of the ball to this acceleration.
What you feel is the force applied to you. The ball's "inertia" relates the net force on the ball, to the ball's acceleration, not to your feelings.

No, that's wrong. If you apply a force to the ball, the ball applies an equal but opposite force to your hand. Whether your force on the ball is balanced by other forces on the ball is irrelevant for this.

What you feel is the force applied to you. The ball's "inertia" relates the net force on the ball, to the ball's acceleration, not to your feelings.
OK, my problem here is that I'm associating an equal reaction force with equilibrium, so I think I see my mistake; my hand and the ball are still in (spatial) equilibrium even thought we are both in motion (through spacetime). So the reaction force makes sense again. I keep having moments like these . . . pardon me.

A.T.
I'm associating an equal reaction force with equilibrium
That is the usual confusion of 2nd and 3rd Newtons Law. The 3rd Law equal but opposite forces are on different objects and don't imply equilibrium for either of them.

https://www.lhup.edu/~dsimanek/physics/horsecart.htm

Hello all

I just joined this forum so forgive me for jumping right in but I have a question about Gravity and the curvature of space time that I can't get answer with a Google search. My question: though I understand that an object remains in orbit because of the curvature of space time and it is this curvature which is responsible for Gravity, but what causes an object that is stationary to fall toward the center of mass if nothing sets it in motion? Does the curvature of space give it a nudge? If so How? Why does a ball which is motionless in my hand fall if I let go of it without giving a push? I understand that if I set it into motion fast enough that it will fall around the earth following the curvature of space but what makes it move toward center of mass if no force is acted on it?
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For me, to understand the basics of General Relativity (GR), I really had to put aside all my previously-conceived notions of position, movement, and acceleration because they are all "relative" concepts.

The only absolutes that I can really accept (now) is whether an object has a net external force on it (that is, it is existing non-inertially) or that object has no net external force on it (that is, it is existing inertially). Those qualities can always be measured as an absolute - and that cannot be said for position or any change in position (with time) for any object on its own. Position and change in position (with time) can only be assigned as relative quantities between two (or more) objects.

Note that some people say that an object is "travelling inertially" or "travelling non-intertially". To me, the word "travelling" is a relative term so I prefer not to use it.

In GR, whether the Moon is in orbit around Earth (in your first case), or the ball is stationary (the moment after it has been released) above the Earth (your second case), both the Moon, the Earth (including your mass), and the ball (assuming it's not touching you) have no external forces on them. That is, the Moon, the Earth, and the ball (in this case) are all intertial.

So, what causes the Moon and the Earth to travel relatively as they do? What causes the ball and the Earth to approach each other as soon as the ball has been released (in your second case)? Well, that is determined by the initial conditions (relative positions and velocities of the objects in question) and the shape (curvature) of spacetime in their vicinity. The Einstein Field Equations give the exact answers.

The other interesting thing about GR is what happens, in your second case, when the ball is in contact with either you or the Earth. When this is happening, you, the ball, and the Earth are part of one larger single object. This new bigger object (you+ball+Earth) exists intertially (i.e., no external forces on it as a whole). However, if you examine pieces of this new bigger object, you, the ball, and the chunk of Earth that doesn't include you and your ball now DO have a net external force on them. In fact, assuming that your mass and the ball's mass are small compared with the rest of the Earth, the direction of the net force on you and the ball is pointing away from the center of mass of the Earth+you+ball as a whole. In fact, this is the case for you right now. The only net force on your body is on your butt, and that force is pointing away from the center of mass of the Earth. This is what Einstein is talking about in his Equivalence Principle.

To answer your second question - to give an answer to "Why does a ball which is motionless in my hand fall if I let go of it without giving a push?", look to the Equivalence Principle. When you're holding on to the ball, there is a net force (upwards) on your feet, and there is also a net force upwards on the the ball. You and the ball have no relative motion. As soon as you let go of the ball, the net force on that ball goes to zero (ignoring air resistance). So now, your feet have a net force (pointing upwards) and the ball has no net external force on it. Of course, there will now be relative motion between you and the ball. Try this experiment when you and your ball are on a rocket ship with its engine turned on. You will get the equivalent result - just as Einstein predicts in his Equivalence Principle.

Bob

Don't forget you and the ball are pushing towards the Earth a tiny bit.
As you release the ball you push slightly less at your feet and as well as fluid resistance or aerodynamics, which is flowing through the air, also a pressure gradient to traverse, and some slight torque involved in the rotation of the Earth propelling it forward rotational direction as it approaches the axis a.k.a. angular momentum.

That is basically the Newtonian explanation, with "curvature of space-time" randomly thrown in. In General Relativity there is no "gravitational force", that opposes the force of the hand. That's why the ball experiences proper-acceleration upwards, when held in the hand.

Sorry A. T. but I have to disagree with the statement that the "curvature of space-time is randomly thrown in." Einstein"s field equations reduce down to the Newtonian equation of F = G(M1)(M2)/R^2. So Newton's equation is valid and connected to general relativity. Newton couldn't explain why his equation worked since he developed it from scientific observation and experimentation. He didn't realize that the R squared term in the denominator is actually the result of multiplying the curvature of space-time squared ( 1/R)^2 times G(M1)(M2). Newton's equation works because of the curvature of space-time. I know the conventional why of teaching this equation is that R is the distance between the centers of the two masses M1 and M2. Also, F= ma =G(M1)(M2)/R^2 where a equals the gravitational acceleration constant g.

A.T.
So Newton's equation is valid and connected to general relativity.
The two models yield similar results in some cases. But the OP asked for an explanation based on general relativity, not on Newtonian force of gravity.

PeterDonis
Mentor
2019 Award
the R squared term in the denominator is actually the result of multiplying the curvature of space-time squared ( 1/R)^2
1/R^2 is not the curvature of spacetime squared. (Purely in terms of units, curvature has units of 1 / length^2, not 1/length, so 1/R^2 has the units of curvature, not curvature squared. But just having the right units is not enough.) If we take a 2-sphere at radius R around a gravitating body, 1/R^2 is the intrinsic curvature of that 2-sphere. (Note that R is not the actual physical distance to the center of the body; that distance is larger than R.) But that is not the same as the curvature of spacetime.

Spacetime curvature is given by the Riemann curvature tensor, which is not a single number; it has 20 independent components in general, and in vacuum it has 10. A typical component in the vacuum surrounding a gravitating body is of order ##M / r^3## in geometric units (or ##GM / c^2 r^3## in conventional units), and describes the tidal gravity produced by the body.

Hello all

I just joined this forum so forgive me for jumping right in but I have a question about Gravity and the curvature of space time that I can't get answer with a Google search. My question: though I understand that an object remains in orbit because of the curvature of space time and it is this curvature which is responsible for Gravity, but what causes an object that is stationary to fall toward the center of mass if nothing sets it in motion? Does the curvature of space give it a nudge? If so How? Why does a ball which is motionless in my hand fall if I let go of it without giving a push? I understand that if I set it into motion fast enough that it will fall around the earth following the curvature of space but what makes it move toward center of mass if no force is acted on it?

Hi Christine,

when you release a ball that is about to fall down, its initial direction will make it to follow a particular spacetime curve (=geodesic) that is different of the one that would correspond to an orbit (which, in fact, is a curve associated to another geodesic). It is wrong to speak of "force" because in essence, we are only allowed to speac of matter-energy on one hand, and space-time distorsions of the other hand . There is no "force" and it's better to avoid this word to avoid confusion. HOWEVER, in the approximation of weak tensor fields, it is possible to prove that Einstein's equations reduce to a form with has the structure of Newton's Law. This basically means that Newton's Law F=ma can be seen as a reinterpretation of weak-field limit of Einstein's equations.

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Ah, too late for the party but it clicked with me like this (please someone point if it's wrong).

Not only space is curved but time as well. And time does move forward.
So if curved surface of Earth is space in one dimension and time in other, and we are standing still some distance away at equator, and times keeps pushing us forward, we will 'accelerate' towards each other.

stevendaryl
Staff Emeritus