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Action and Reaction

  1. Jul 6, 2013 #1
    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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  3. Jul 6, 2013 #2


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    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.
  4. Jul 6, 2013 #3


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    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.
  5. Jul 7, 2013 #4
    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'.
  6. Jul 7, 2013 #5
    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...

    not "reactionary."

    This link explains in more detail.

    Last edited: Jul 7, 2013
  7. Jul 7, 2013 #6


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    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.
  8. Jul 7, 2013 #7


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    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.
  9. Jul 7, 2013 #8
    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?
  10. Jul 7, 2013 #9
    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?
  11. Jul 7, 2013 #10
    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?

  12. Jul 7, 2013 #11


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    The underlying mechanism is described by the Einstein field equations (EFE):

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

    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.

    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.

    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.
  13. Jul 7, 2013 #12


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    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.
  14. Jul 7, 2013 #13
    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?

    Thanks in advance.
    Last edited: Jul 7, 2013
  15. Jul 7, 2013 #14
    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.
  16. Jul 7, 2013 #15


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    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.
  17. Jul 7, 2013 #16


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    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'.

    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).
    Last edited: Jul 7, 2013
  18. Jul 7, 2013 #17
    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.
  19. Jul 7, 2013 #18


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    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.
  20. Jul 7, 2013 #19


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    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.
  21. Jul 7, 2013 #20


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    Curved spacetime not just space.

    This is explained here for an apple falling from rest:

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