# Action and Reaction

## Main Question or Discussion Point

Action and reaction are equal and opposite.
So, when mass (matter) acts on space and bends it, why doesn’t space react to this action in any detectable way?

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mfb
Mentor
Action and reaction are equal and opposite.
Between objects, in Newtonian physics.
Space is not an object, and General Relativity is not Newtonian physics. Why do you expect this concept here?

Matter bends space, space influences the motion of matter.

Dale
Mentor
Action and reaction are equal and opposite.
So, when mass (matter) acts on space and bends it, why doesn’t space react to this action in any detectable way?
In the context of Newton's 3rd law "action" and "reaction" are forces. When spacetime bends I am not aware of any sense in which there is a force acting on spacetime.

In the context of Newton's 3rd law "action" and "reaction" are forces. When spacetime bends I am not aware of any sense in which there is a force acting on spacetime.
Then, what is the underlying mechanism for this 'bending' to happen?

In other words, If mass reacts with space-time and it does not do so through some kind of 'force', then 'how' does mass do this (lead to this bending of space and time).

Plz be patient. I have thought long-long on this and have come to the thought that there must be some kind of 'physical connection' between mass and space-time that could be detected.

Though, some of my friends say that such things can be explained only by mathematics. I don't vouch for this idea as mathematics only describes some kind of underlying 'physical mechanism'.

Plz be patient. I have thought long-long on this and have come to the thought that there must be some kind of 'physical connection' between mass and space-time that could be detected.
There is, its called the space-time curvature that is gravity. Einstein's field equations, from my interpretation, do not imply causation so much as they imply a "relation" or correlation. This is in the spirit of Einstein's universe, which is said to be "background independent," versus Netwton's universe, which is said to be "background dependent." In a background dependent universe, objects are actors in a play which is carried out in on a stage which is an unchanging space-time background. In Einstein's universe, space-time evolves along with the characters in the play. These are "relational" properties of space-time and mass...

If mass reacts with space-time and it does not do so through some kind of 'force', then 'how' does mass do this (lead to this bending of space and time).
not "reactionary."

This link explains in more detail.

http://www.einstein-online.info/spotlights/background_independence/?set_language=en

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WannabeNewton
Newtonian gravity can also be made background independent; Newtonian gravity can be reformulated in a generally covariant way so that it too is a manifestation of space-time curvature and so that the space-time geometry is dynamical. The differences between general relativity and Newtonian gravity, once cast in a geometric form, are more specific.

General relativity does not explain why mass-energy induces space-time curvature; it simply tells us how. There is a difference.

1 person
BruceW
Homework Helper
In other words, If mass reacts with space-time and it does not do so through some kind of 'force', then 'how' does mass do this (lead to this bending of space and time).

Plz be patient. I have thought long-long on this and have come to the thought that there must be some kind of 'physical connection' between mass and space-time that could be detected.
Force is a classical concept. In quantum theory, there are no 'classical forces', there are only 'interactions'. If you are happy with this idea of 'interactions', then there is hope for a quantum theory of gravity one day, which would mean that general relativity could be thought of as resulting from a large number of quantum-mechanical interactions.

In the context of Newton's 3rd law "action" and "reaction" are forces. When spacetime bends I am not aware of any sense in which there is a force acting on spacetime.
Matter interacts with space-time?

What do these 'interactions' involve if not 'force' and other such classical concepts? Is such an agency yet to be discovered or simply it's undiscoverable?

Let's talk about space in a nucleus. Is this space also bent? With so much concentration of matter in a nucleus (and so much of space warping) how could it's constituents move at all?

Between objects, in Newtonian physics.
Space is not an object, and General Relativity is not Newtonian physics. Why do you expect this concept here?

Matter bends space, space influences the motion of matter.
I expect this concept here because it is known now that quantum fluctuations of space lead to production of material particles. So why not suppose material properties for space?

General relativity does not explain why mass-energy induces space-time curvature; it simply tells us how. There is a difference.
GR tells us about how much mass would produce how much 'bending' and things like that but it does not tell about 'how' this phenomenon takes place i.e. the mechanism of this phenomenon is not explained. The 'why' question is even more difficult.

Let me give a very foolish example ( but it would convey what I want to say).

Suppose, there is a planet in a region of the universe where there is no space (it's outright wrong I know, but still bear with me.) Now, this planet is slowly moved towards a region that has space. Suppose it is 2 light years away from the boundary of this region that has space.

Will this planet bend space from such a distance?

Now, it's moved closer by 1 light year, will the space bend?

Will the effect of this planet travel faster than light and bend the space at once or will it take 1 full year before the effect of this planet (mass) reaches the region of space and bend it?

YOU ARE FREE TO LAUGH AT ME

Dale
Mentor
Then, what is the underlying mechanism for this 'bending' to happen?
The underlying mechanism is described by the Einstein field equations (EFE):
https://en.wikipedia.org/wiki/Einstein_field_equations

Matter and fields have a stress-energy tensor, and this tensor is proportional to the curvature tensor.

Though, some of my friends say that such things can be explained only by mathematics. I don't vouch for this idea as mathematics only describes some kind of underlying 'physical mechanism'.
In this case, I agree with both you and your friends. The mathematics indeed are only a description of the physical mechanism, not the physical mechanism itself, but so is any other set of words or human symbols that I could put together. I could give different aspects of the mechanism names, but those names are not the mechanism only a description. I could make analogies between the mechanism and other things, but those analogies are not the mechanism either. It turns out that the math is the most accurate and least biased description of the physical mechanism that we have available.

What do these 'interactions' involve if not 'force' and other such classical concepts?
Why would there be a force involved? Spacetime doesn't have a mass and it doesn't have an acceleration, so why should there be any force involved? Your assumption seems strange to me.

Is such an agency yet to be discovered or simply it's undiscoverable?

Let's talk about space in a nucleus. Is this space also bent? With so much concentration of matter in a nucleus (and so much of space warping) how could it's constituents move at all?
It is quite possible that a working theory of quantum gravity will answer these two questions. Currently we do not have such a theory, but when we do it will also be mathematical in nature. I.e. it will explain the EFE as an approximation to the mathematical equations of a more complete theory.

1 person
BruceW
Homework Helper
Let me give a very foolish example ( but it would convey what I want to say).

Suppose, there is a planet in a region of the universe where there is no space (it's outright wrong I know, but still bear with me.) Now, this planet is slowly moved towards a region that has space. Suppose it is 2 light years away from the boundary of this region that has space.

Will this planet bend space from such a distance?

Now, it's moved closer by 1 light year, will the space bend?

Will the effect of this planet travel faster than light and bend the space at once or will it take 1 full year before the effect of this planet (mass) reaches the region of space and bend it?
space is everywhere. But yes, the effect of the planet on bending space can only travel at a maximum speed of c. https://www.physicsforums.com/showthread.php?t=699522

Also, going back to why there is no force... I think once you learn a bit more about general relativity, you will start to see what it is all about. I'll try to give a first explanation. OK, so in General relativity, we allow spacetime to be curved. And Einstein's clever insight is that the force of gravity can't be distinguished from a curvature of spacetime. Therefore, we assume that gravity is not a force, but that it comes about due to a curvature of spacetime. Now, if we have a test mass (say a person) with zero forces acting on him, then from Netwon's laws, we would say he moves in a straight line. But because we are now allowing curved spacetime, there is no such thing as a straight line anymore. We have to generalize to the concept of geodesics. So he moves along a geodesic. This takes into account the curvature of spacetime. And the curvature of spacetime depends on mass and energy in a fairly straightforward way, which reduces to the 'old' concept of gravity in the limit of slow speeds and small curvature.

space is everywhere. But yes, the effect of the planet on bending space can only travel at a maximum speed of c. https://www.physicsforums.com/showthread.php?t=699522

Also, going back to why there is no force... I think once you learn a bit more about general relativity, you will start to see what it is all about. I'll try to give a first explanation. OK, so in General relativity, we allow spacetime to be curved. And Einstein's clever insight is that the force of gravity can't be distinguished from a curvature of spacetime. Therefore, we assume that gravity is not a force, but that it comes about due to a curvature of spacetime. Now, if we have a test mass (say a person) with zero forces acting on him, then from Netwon's laws, we would say he moves in a straight line. But because we are now allowing curved spacetime, there is no such thing as a straight line anymore. We have to generalize to the concept of geodesics. So he moves along a geodesic. This takes into account the curvature of spacetime. And the curvature of spacetime depends on mass and energy in a fairly straightforward way, which reduces to the 'old' concept of gravity in the limit of slow speeds and small curvature.
From the foolish example that I gave, it seems to my mind that there is 'something' that emanates or oozes out of mass, which then warps space. this is just a vague thought.

Anyhow, a question has come to my mind.

Suppose, there is a planet which has curved space around it. A ball is placed in this space ( I push it from a roof).

Why does it move towards the planet in the very first place when there is no force acting on the ball by way of gravitation.

If someone says that the gravity of the planet attracts the ball towards itself, the scenario is perfectly understandable. But as per relativity, when gravity is just the curving of space, what makes the ball fall downward. if you say, I gave it a force by pushing it, why does its speed increase all the way down and not remain proportional to the push that I gave to the ball?

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Suppose, there is a planet which has curved space around it. A ball is placed in this space ( I push it from a roof).

Why does it move towards the planet in the very first place when there is no force acting on the ball by way of gravitation.

If someone says that the gravity of the planet attracts the ball towards itself, the scenario is perfectly understandable. But as per relativity, when gravity is just the curving of space, what makes the ball fall downward.
Suppose two people start from different points on the equator of the Earth and walk straight north. Eventually, they will collide at the north pole. Why did they collide, when they were always walking straight and they started out on parallel paths?

The same sort of thing is going on with gravity in general relavity. The curvature of spacetime means that objects following straight paths through spacetime actually end up being attracted to each other. There is no force of gravity in GR. The ball just moves in the straightest possible path through spacetime. The straightest possible path happens to intersect the surface of the planet.

Nugatory
Mentor
If someone says that the gravity of the planet attracts the ball towards itself, the scenario is perfectly understandable. But as per relativity, when gravity is just the curving of space, what makes the ball fall downward. if you say, I gave it a force by pushing it, why does its speed increase all the way down and not remain proportional to the push that I gave to the ball?
As long as all the same forces are acting on you and the ball(which means you have to be free-falling along with the ball), the speed of the ball relative to you will remain constant and proportional to the push you gave the ball. Don't be confused by the way that the surface of the earth is accelerating towards you and the ball.

BruceW
Homework Helper
From the foolish example that I gave, it seems to my mind that there is 'something' that emanates or oozes out of mass, which then warps space. this is just a vague thought.
uh, not quite. general relativity is a local phenomena, in a similar way to how electromagnetism is a local phenomena. In electromagnetism, if we know the charge distribution at a point, then the equations of electromagnetism tell us something about the electric field at that point. And the electric field far from that point is not immediately affected. A similar thing happens in general relativity, but instead of an electric field, we have the metric tensor (which contains information about curvature of space, e.t.c.) itself which mediates gravitational phenomena. So it is the properties of space itself that is 'oozing out'.

Naveen3456 said:
Suppose, there is a planet which has curved space around it. A ball is placed in this space ( I push it from a roof).

Why does it move towards the planet in the very first place when there is no force acting on the ball by way of gravitation.

If someone says that the gravity of the planet attracts the ball towards itself, the scenario is perfectly understandable. But as per relativity, when gravity is just the curving of space, what makes the ball fall downward. if you say, I gave it a force by pushing it, why does its speed increase all the way down and not remain proportional to the push that I gave to the ball?
Not sure what you mean here. You seem to imply that a ball falling in curved space would not pick up speed. But surely the ball would pick up speed.

edit: now I think I see what you mean. you mean that since the ball is falling in the generalization of a straight line (a geodesic), then why is it's velocity changing? It is as Nugatory says, it is because you (on the roof) have forces acting on you, and you have some arbitrary path through space. So from your perspective, objects moving along a geodesic could have any kind of motion. (remember that motion is relative).

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Not sure what you mean here. You seem to imply that a ball falling in curved space would not pick up speed. But surely the ball would pick up speed.
Plz explain why are you so sure about the ball picking speed when no force is present that would increase the speed of the ball. Of course I gave force to the ball by pushing it, but it was a small push of hand ( a small force) which cannot result in the high speed with which the ball hits the ground after falling from the roof.

BruceW
Homework Helper
Newton's first law does not work, since we are using general relativity, and there is a planet, so there is no 'inertial reference frame'. So even though the concept of a geodesic is similar in some ways to a straight line, it is not similar in other ways. The main thing to keep in mind is that "a test object with no forces acting on it moves along a geodesic". This is how it is similar to Newton's "a test object with no forces acting on it moves along a straight line". It may not be similar in other ways.

DrGreg
Gold Member
Plz explain why are you so sure about the ball picking speed when no force is present that would increase the speed of the ball. Of course I gave force to the ball by pushing it, but it was a small push of hand ( a small force) which cannot result in the high speed with which the ball hits the ground after falling from the roof.
The difference between Einstein's theory and Newton's is that where Newton says the ball is accelerated downwards towards the ground, Einstein says the ground is accelerated upwards towards the ball! So there is no force acting on the ball, causing it to accelerate down. There is a force acting on the ground causing it to accelerate up. The curvature of spacetime (note: spacetime, not space) is necessary to explain how the surface of the Earth can be accelerating outward yet it isn't expanding.

A.T.
Suppose, there is a planet which has curved space around it.
Curved spacetime not just space.

A ball is placed in this space ( I push it from a roof).Why does it move towards the planet in the very first place when there is no force acting on the ball by way of gravitation.
This is explained here for an apple falling from rest:

BruceW
Homework Helper
yeah, it should be spacetime, not space. I am also guilty of using the word 'space' when I mean 'spacetime' hehe. Partly because I've been reading a lot about cosmological models, where time is effectively treated in the Newtonian way. But terminology is important! Or it will all start to get very confusing very quickly.

Dale
Mentor
From the foolish example that I gave, it seems to my mind that there is 'something' that emanates or oozes out of mass, which then warps space. this is just a vague thought.
The EFE are local equations, so there is no need for anything to ooze out. Each bit of stress-energy just bends spacetime in its local vicinity.

Why does it move towards the planet in the very first place when there is no force acting on the ball by way of gravitation.
Remember that it is spacetime which is curved, not just space. A ball "at rest" is still moving through time. In an earth-fixed coordinate system a geodesic which is initially purely in the time direction will gradually curve into the space direction.

EDIT: I see that both of these points have already been made above. At least the message is consistent.

Einstein says the ground is accelerated upwards towards the ball! So there is no force acting on the ball, causing it to accelerate down. There is a force acting on the ground causing it to accelerate up. .
Can you take pains to explain these lines, without any mathematics, of course?

As long as all the same forces are acting on you and the ball(which means you have to be free-falling along with the ball), the speed of the ball relative to you will remain constant and proportional to the push you gave the ball. Don't be confused by the way that the surface of the earth is accelerating towards you and the ball.
To my small mind/brain, it is still not at all clear as to why the speed of the ball should increase on falling from the roof when there is nothing called as 'gravitational force?

Nugatory
Mentor
To my small mind/brain, it is still not at all clear as to why the speed of the ball should increase on falling from the roof when there is nothing called as 'gravitational force?
The speed of the ball doesn't increase. The surface of the earth is moving towards the ball at an ever-increasing speed (until they collide).