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Gravity not a force

  1. Sep 17, 2012 #1
    I read somewhere that gravity is not a force. Is this true? What does it mean?
     
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  3. Sep 17, 2012 #2

    rbj

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    according to the theory of General Relativity, there is an equivalence to a body falling in free fall to an identical body out in free space traveling at a constant velocity.

    what a big mass (like a planet) does is warp or curve space in such a way so that objects flying about freely in this curved space appears to us in our Euclidian space to be following a curve where it just appears that a force is acting on the body that causes its path to be curved toward the center of that big mass.
     
  4. Sep 17, 2012 #3
    where did you read
     
  5. Sep 17, 2012 #4
    Actually, in a certain sense, it isn't.
     
  6. Sep 17, 2012 #5

    sophiecentaur

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    For all practical purposes, we can say that Gravity 'produces' a force - because we can all feel it. Modern Physics takes matters further than that, of course, but the same 'force' effects are observable even when they are 'explained' slightly differently.
     
  7. Sep 17, 2012 #6

    K^2

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    There is a class of forces in physics known as fictitious forces or inertial forces. Whether they act as real force or not depends on your choice of coordinate system. If you chose surface of the Earth as your reference point, for example, you have to account for the force of gravity for all of your equations to make sense. But if you happen to be inside an elevator during a free-fall, you will not experience gravity. For all intents and purposes, gravity just goes away.

    This is very similar to centrifugal force. It is also a fictitious force. If you are observing a rotating space station from an inertial frame, you can describe everything that happens in terms of forces of interaction between various objects. But if you are standing inside the station, measuring all positions relative to the station, and you don't take rotation into account, it appears that there is another force, very similar to gravity, that keeps pushing you away from center of the station.
     
  8. Sep 17, 2012 #7

    BruceW

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    In general relativity, they don't count gravity as a force, but its effects are taken into account in the equations of the spacetime curvature. In the limit of low speed and weak curvature, we can say that gravity acts approximately like a force (which is the physics that was around before Einstein's relativity).
     
  9. Sep 18, 2012 #8
    I think physicists don't get too worked up about what exactly constitutes a force. Depending on the context, they will call gravity a fundamental force or a fictitious force. But it's not something to argue over. They'll just use the term that is more convenient at the time. Being too concerned with the terminology is just stupid, since physicists will agree on what gravity _does_.

    So if one textbook says gravity is a force and another one says it's a figment of the coordinate system, it doesn't mean that one textbook is wrong and one is right. We are all just working with human-defined models and it's the observable phenomena that really count.

    In GR, gravity is not a force, but in everything else, gravity is a force.
     
  10. Sep 18, 2012 #9

    D H

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    No, you can't. Newtonian POV: We can feel every real force but gravity. There is no way to directly sense the gravitational force. However, note that we don't "feel" fictitious forces.

    General relativity POV: We feel every real force, period. Gravitation falls into the real of fictitious forces in general relativity. That gravitation can't be sensed -- no surprise. We can't sense any of the fictitious forces.


    Put an accelerometer at rest on top of a table. That accelerometer will register an acceleration of 1g pointing upward, yet it's obviously not accelerating. It's standing still. That 1g upward acceleration that it is registering is the normal force exerted by the table on the accelerometer. Now push the accelerometer off the table. In free fall, the accelerometer registers an acceleration of near zero. Yet it obviously *is* accelerating. The gravitational force acting on the accelerometer hasn't changed. What has changed is that the normal force is now absent. The accelerometer doesn't measure gravity because there is no way to measure gravity.

    The same goes for what you feel. You aren't feeling gravity. What you are feeling is the ground pushing up on your feet, your skeleton transmitting this upward force to the less rigid parts of your body, and eventually, your inner ear. Your inner ear is equipped with a natural accelerometer. It senses that upward acceleration.

    Now imagine taking one of those zero g amusement park rides, or being a passenger on NASA's Vomit Comet. During those intervals of zero g, your stomach and your inner ear rebel at the loss of this upward acceleration that is normally sensed when you are standing on the ground. You aren't feeling those forces during those intervals of zero g.


    Exactly. Worrying about what different theories call things is just a semantics game. What really matters is how well physics predicts experimental outcomes. In those realms where Newtonian mechanics is (approximately) valid, Newtonian mechanics and relativity will agree on experimental outcomes, sensor readings, etc.
     
  11. Sep 18, 2012 #10
    This is the sort of thing laymen hear on Carl Sagan programs. Actually, I suspect that most professional physicists would get it wrong.

    If you have a box falling in the Earth's gravitational field and you shine a light horizontally, the light will be deflected downwards with an acceleration of around 19.6 meters per second squared. the doubling is because in Relativity there are gravitational forces that go as the square of the velocity. So the box is falling at around 9.8 meters per secomnd squared and the light is falling at double thatr--it therefore is possible to distinguish between the box in free fall vs a box at rest without gravity.


    The is clearly incorrect. If you are standing here on Earth you are experiencing gravity, and as you yourself said, gravity invoves spacetime not being Euclidian. So we are not in "our Euclidian space".
     
  12. Sep 18, 2012 #11
    I agree with that - the "force of gravity" acts on a spring scale when you stand on it. The "different explanation" is to say that that force is not due to gravitation but due to acceleration. And as others said, whatever you call it or how you interpret it, everyone agrees that the spring is compressed by a real force.

    The situation is a lot more hazy when an object falls under the influence of a gravitational field; it then depends on one's exact definition of "force" (as well as on one's definition of "field"!).
     
    Last edited: Sep 18, 2012
  13. Sep 18, 2012 #12
    Yes, the spring is being compressed by a force. GR just says that force is that of the earth pushing upwards on the scale. There is a reason that a stationary accelerometer reads as though the acceleration it experiences is up, not down. This is not sleight-of-hand with jargon to confuse people. In a very material, concrete way, objects cannot and do not know about how gravity affects their motions.
     
  14. Sep 18, 2012 #13
    "I read somewhere that gravity is not a force. Is this true? What does it mean?"

    Simple answer if gravity was not a force we wouldn't be here! Just because we haven't found graviton particle it dosn't mean gravity dosen't exit. 8)
     
  15. Sep 19, 2012 #14

    sophiecentaur

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    The 'purist' view is strongly against allowing gravity to be a force. Fair enough, because a gravitational field 'causes' a force to act on a mass - rather than actually 'being' a force. But why is the same purist view not held so strongly in the case of EM fields? I appreciate that if you were in a charged ship with no windows then you could detect an acceleration due to the presence of an EM field (due to a measurable acceleration). This is different from the gravitational case, of course (an accelerometer wouldn't react because the Equivalence Princple is at work), but in both cases, the acceleration is actually measurable as soon as you can see (or reference) the outside world.
    Why restrict the measuring conditions in order to 'classify' the gravitational effect as being different from the EM effect?
    Weight is a readily perceived and measured force - given the appropriate equipment - so why are we not 'allowed' to treat gravity as a force? It is such a tangible thing that it seems to me that the purist tail is wagging the dog of experience.
     
  16. Sep 19, 2012 #15
    I'm very much a 'purist' and I disagree with what you call the 'purist' view for reasons similar to the ones you mention. :smile:

    It would be useful if opinionated people present their definitions of "force" and "field", and then explain how they reason that those definitions logically lead to their expressed opinions.
     
  17. Sep 19, 2012 #16

    sophiecentaur

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    OK, my "opinion" is that a Force will produce an acceleration or change of shape. (That's Newtonian - based). How one measures these changes is, to my mind, irrelevant.
    For a definition of Field in this context, I'd say that the presence of a Field will produce a Force on an object with a particular property - e.g. mass / charge / current. So F=mG and F=qE for instance. So the Field is force per unit of some property.
    So we could continue the discussion with those definitions - or with others, perhaps(?).
     
  18. Sep 19, 2012 #17
    Imagine two travellers standing at different points along the equator. Now both of them start walking north, towards the north pole. What happens ? The further north they walk, the closer they get to one another ! They approach each other not because of any force acting between them, but because of the geometry of earth's surface. Gravity works the same - as time passes, two bits of matter will gravitate towards each other, not because of any force, but because of the geometry of space-time.
     
  19. Sep 19, 2012 #18

    sophiecentaur

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    If these two travellers stop walking, do they experience anything? Is there any Force acting on them to make them continue getting closer to the Pole? I do understand this is just an analogy but I feel it is too far away from the point about the 'reality' or otherwise of a gravitational force. You would need to say what the equivalent of a force is for these two travellers. It certainly couldn't be the same as the 'force' that pulls two masses together because the analogy is not 1:1.

    I can see that GR tries to explain the origin of a force like the one that the proximity of two masses causes but, if the effect is the same as that which occurs between two charges, then why is it not allowed to be called a force? What distinguishes the one 'force' from the other force apart from the difficulty in detecting it?
     
  20. Sep 19, 2012 #19
    You are right, it isn't at 1:1 analogy. However ( and I failed to mentioned that ), once one understands that the northward direction corresponds to time, and the distance between the travellers corresponds to spatial separation, then the situation is clear : there is a tendency for two bits of matter to approach one another over time, purely based on the geometry of space-time. This is, I believe, quantified via the Raychaudhuri equation.
    The question as to whether you need a force for them to continue going north is rather meaningless, since one cannot stop moving through time.
     
    Last edited: Sep 19, 2012
  21. Sep 19, 2012 #20
    Thanks for the precision!

    If one uses either of those definitions then gravity is a force in GR that appears whenever a gravitational field is assumed. Einstein called in his 1916 GR paper the gravitational field a "field of force [..] which possesses the remarkable property of imparting the same acceleration to all bodies".

    In a reference frame that is attached to the surface of the earth, your acceleration when you stand on a scale is zero so that the force that you feel is fully ascribed to gravitational field (and not acceleration)*.
    Conversely, in a "free falling" GR frame the gravitational field has vanished and that same force is ascribed to acceleration instead.

    *From the more standard ECI frame POV, it is gravity due to the mass of the Earth, which is partly reduced due to the acceleration of the rotating surface of the Earth.
     
    Last edited: Sep 19, 2012
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