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I don't know how "force of gravity" is a wrong belief but "the only force you are actually feeling is the upward force exerted by your own muscles in order to k

I don't know how "force of gravity" is a wrong belief but "the only force you are actually feeling is the upward force exerted by your own muscles in order to keep your arm accelerating continuously away from a straight path in spacetime." is correct. can anyone explain? EDIT: my mind is totally blown by this; it put together some pieces of general relativity in a new way for me today. thanks HN!

vibragiel 16y ago
He is saying this because, in general relativity, gravitation is no longer explained as a force between two masses, but as their "warping" of their surrounding space(time). http://en.wikipedia.org/wiki/Gravitation#General_relativity http://en.wikipedia.org/wiki/Gravitation#General_relativity
- a-priori 16y ago
  This is true, but it doesn't invalidate the description of gravity as being an attractive force between masses. The warping of spacetime is just an explanation of what's happening to create that force.
  - vibragiel 16y ago
    There are no forces involved in Einstein's gravitation. You can see it as a pseudo-force (like Coriolis or centripetal) if you like, for practical (and mental sanity) reasons. But it's not a true force. See "equivalence principle". http://en.wikipedia.org/wiki/Equivalence_principle http://en.wikipedia.org/wiki/Equivalence_principle
    - icegreentea 16y ago
      What happens if we ever detect a graviton? I never really understood this part. General relativity gives us one explanation of gravity (mass curves space-time), while quantum theory tells us that gravity arises from the exchange of particles. So... how do we reconcile? I imagine if we ever discover a unified theory, it'll manage to deal with that. But how does say string theory (for lack of other examples) reconcile these two views? ...or is it something as simple as the exchange of particles causes the warping of space >.>
      - kscaldef 16y ago
        > quantum theory tells us that gravity arises from the exchange of particles No, it doesn't. That statement is wrong on two levels: 1) We don't have a confirmed quantum theory of gravity 2) In quantum field theories in general it's not really accurate to say that "[forces] arise from the exchange of particles". In some QFTs, in some situations, you can perform a perturbative expansion which maps onto the Feynman diagram view of particle exchange, but that's not universally possible. They really are quantum _field_ theories, not quantum particle theories.
  - jessriedel 16y ago
    vibragiel is basically right, but I'll just comment here that if you think hard about what you are arguing, it is semantics. A physicist will define a force as an object's deviation from a geodesic (straight line motion in the relevant geometry). In this case, gravity is not a force as objects attracted by gravity are indeed following geodesics. On the other hand, if you define a force as an object's deviation caused by a second object from the path that would be taken were the second object not there, then gravity is indeed still a force. In this case, the second object is changing the geometry itself and, hence, the associate geodesic.
    - a-priori 16y ago
      It is semantics, and all comes down to what's a 'force' and what isn't in theoretical physics. I was arguing from the second definition. The first definition is tautological: First, you define a curved space, accounting for gravitational forces such that a small object will follow a certain geodesic. Then, you say that since it does indeed follow said geodesic in that geometry without deviation, gravity is not a force. Well of course not! With the same same hand-wavy argument, you could define the curvature of space based on electromagnetic field strength, for example, and argue that electromagnetism is not a force.
      - deleted 16y ago
        [deleted]
      - jessriedel 16y ago
        > The first definition is tautological >With the same same hand-wavy argument, you could define the curvature of space based on electromagnetic field strength, for example, and argue that electromagnetism is not a force. No. You're very wrong here. You (irritatingly) seem to think that because you have only been exposed to popularized hand-wavy arguments, that physicists necessarily rely on them. The vast majority of possible forces (including Newtonian gravity and electromagnetism) can not be explained the way gravity is explained. Your confusion probably comes from not understanding how much smaller the space of possible geometries is than the space of possible forces. In fact, the stunningly beautiful thing about gravity is that it can be derived from knowing only that it is a geometric effect and obeys a few symmetries. Even if you relax some of these symmetries (e.g. Brans-Dicke theory) the resulting parameterizable space of possible geometries is very restricted.
kscaldef 16y ago
It's the same way that there's not really such a thing as centrifugal force. It a fictitious force that you perceive as a result of being in an accelerating frame of reference.
- deleted 16y ago
  [deleted]
- pbhjpbhj 16y ago
  I'm never in an accelerating frame of reference. When I walk I push the earth around, and drag the universe along with it a little.
michael_nielsen 16y ago
The "force" of gravity is not a fundamental concept in general relativity. Here's a way of thinking about what's going on. Consider a large mass, like the Earth. That mass curves spacetime in the vicinity of the Earth. A small object, like a satellite, simply moves completely freely in a "straight line" (i.e., geodesic path) according to that curved geometry. The curved geometry is such that those geodesics are just the satellite orbits that we see. In other words, the satellite isn't affected by any "force", it's just moving in a straight line in a geometry that happens to be curved, so we see it doing circles around the Earth. It's exceedingly neat: in John Wheeler's great phrase, matter tells spacetime how to curve, and spacetime tells matter how to move. No forces required! Of course, after the fact you can tack on a notion of "force", but it's in no way fundamental. Same thing is going on with projectile motion here on Earth. Once you internalize this point of view, the statement about holding your arm in place becomes a lot clearer. The "natural" force-free thing your arm wants to do is to move along geodesics of spacetime, which means falling toward the Earth at an acceleration of g. But if we exert a (muscular) force to keep it up, then we can hold it in place. In short, in this point of view, forces are things which cause deviations from geodesic motion.
- zach 16y ago
  As far as simple visual illustrations go, this is the best I've seen: http://www.youtube.com/watch?v=O-p8yZYxNGc#t=6m31 http://www.youtube.com/watch?v=O-p8yZYxNGc#t=6m31
  - natep 16y ago
    What's always bothered me about these kinds of animations is that there is a grid of straight lines, but none of the objects actually follow these grid lines. Instead, the geodesics they follow are determined by the topology of the deformed surface, invoking the viewer's intuitive understanding of gravity...in order to understand the unintuitive formulation of it. But, as you say, I haven't seen better.
- iwr 16y ago
  While the notion is simple, the math is not.
henrikschroder 16y ago
If you are standing on the ground, the only actual force is the normal force of the ground acting on your feet to stop you from falling into the center of the Earth. However, it feels like you are pushing down on the Earth, but that's technically a pseudo-force. Another way of looking at it is to imagine that you are standing in an elevator in space that is accelerating with the speed of 1g. That scenario will feel exactly the same as if that elevator was standing still on the surface of the Earth, but it's more clear that the elevator is pushing you, and not the other way around.

vibragiel 16y ago

He is saying this because, in general relativity, gravitation is no longer explained as a force between two masses, but as their "warping" of their surrounding space(time). http://en.wikipedia.org/wiki/Gravitation#General_relativity http://en.wikipedia.org/wiki/Gravitation#General_relativity

a-priori 16y ago
This is true, but it doesn't invalidate the description of gravity as being an attractive force between masses. The warping of spacetime is just an explanation of what's happening to create that force.
- vibragiel 16y ago
  There are no forces involved in Einstein's gravitation. You can see it as a pseudo-force (like Coriolis or centripetal) if you like, for practical (and mental sanity) reasons. But it's not a true force. See "equivalence principle". http://en.wikipedia.org/wiki/Equivalence_principle http://en.wikipedia.org/wiki/Equivalence_principle
  - icegreentea 16y ago
    What happens if we ever detect a graviton? I never really understood this part. General relativity gives us one explanation of gravity (mass curves space-time), while quantum theory tells us that gravity arises from the exchange of particles. So... how do we reconcile? I imagine if we ever discover a unified theory, it'll manage to deal with that. But how does say string theory (for lack of other examples) reconcile these two views? ...or is it something as simple as the exchange of particles causes the warping of space >.>
    - kscaldef 16y ago
      > quantum theory tells us that gravity arises from the exchange of particles No, it doesn't. That statement is wrong on two levels: 1) We don't have a confirmed quantum theory of gravity 2) In quantum field theories in general it's not really accurate to say that "[forces] arise from the exchange of particles". In some QFTs, in some situations, you can perform a perturbative expansion which maps onto the Feynman diagram view of particle exchange, but that's not universally possible. They really are quantum _field_ theories, not quantum particle theories.
- jessriedel 16y ago
  vibragiel is basically right, but I'll just comment here that if you think hard about what you are arguing, it is semantics. A physicist will define a force as an object's deviation from a geodesic (straight line motion in the relevant geometry). In this case, gravity is not a force as objects attracted by gravity are indeed following geodesics. On the other hand, if you define a force as an object's deviation caused by a second object from the path that would be taken were the second object not there, then gravity is indeed still a force. In this case, the second object is changing the geometry itself and, hence, the associate geodesic.
  - a-priori 16y ago
    It is semantics, and all comes down to what's a 'force' and what isn't in theoretical physics. I was arguing from the second definition. The first definition is tautological: First, you define a curved space, accounting for gravitational forces such that a small object will follow a certain geodesic. Then, you say that since it does indeed follow said geodesic in that geometry without deviation, gravity is not a force. Well of course not! With the same same hand-wavy argument, you could define the curvature of space based on electromagnetic field strength, for example, and argue that electromagnetism is not a force.
    - deleted 16y ago
      [deleted]
    - jessriedel 16y ago
      > The first definition is tautological >With the same same hand-wavy argument, you could define the curvature of space based on electromagnetic field strength, for example, and argue that electromagnetism is not a force. No. You're very wrong here. You (irritatingly) seem to think that because you have only been exposed to popularized hand-wavy arguments, that physicists necessarily rely on them. The vast majority of possible forces (including Newtonian gravity and electromagnetism) can not be explained the way gravity is explained. Your confusion probably comes from not understanding how much smaller the space of possible geometries is than the space of possible forces. In fact, the stunningly beautiful thing about gravity is that it can be derived from knowing only that it is a geometric effect and obeys a few symmetries. Even if you relax some of these symmetries (e.g. Brans-Dicke theory) the resulting parameterizable space of possible geometries is very restricted.

kscaldef 16y ago

It's the same way that there's not really such a thing as centrifugal force. It a fictitious force that you perceive as a result of being in an accelerating frame of reference.

deleted 16y ago
[deleted]
pbhjpbhj 16y ago
I'm never in an accelerating frame of reference. When I walk I push the earth around, and drag the universe along with it a little.

michael_nielsen 16y ago

The "force" of gravity is not a fundamental concept in general relativity. Here's a way of thinking about what's going on. Consider a large mass, like the Earth. That mass curves spacetime in the vicinity of the Earth. A small object, like a satellite, simply moves completely freely in a "straight line" (i.e., geodesic path) according to that curved geometry. The curved geometry is such that those geodesics are just the satellite orbits that we see. In other words, the satellite isn't affected by any "force", it's just moving in a straight line in a geometry that happens to be curved, so we see it doing circles around the Earth. It's exceedingly neat: in John Wheeler's great phrase, matter tells spacetime how to curve, and spacetime tells matter how to move. No forces required! Of course, after the fact you can tack on a notion of "force", but it's in no way fundamental. Same thing is going on with projectile motion here on Earth. Once you internalize this point of view, the statement about holding your arm in place becomes a lot clearer. The "natural" force-free thing your arm wants to do is to move along geodesics of spacetime, which means falling toward the Earth at an acceleration of g. But if we exert a (muscular) force to keep it up, then we can hold it in place. In short, in this point of view, forces are things which cause deviations from geodesic motion.

zach 16y ago
As far as simple visual illustrations go, this is the best I've seen: http://www.youtube.com/watch?v=O-p8yZYxNGc#t=6m31 http://www.youtube.com/watch?v=O-p8yZYxNGc#t=6m31
- natep 16y ago
  What's always bothered me about these kinds of animations is that there is a grid of straight lines, but none of the objects actually follow these grid lines. Instead, the geodesics they follow are determined by the topology of the deformed surface, invoking the viewer's intuitive understanding of gravity...in order to understand the unintuitive formulation of it. But, as you say, I haven't seen better.
iwr 16y ago
While the notion is simple, the math is not.

henrikschroder 16y ago

If you are standing on the ground, the only actual force is the normal force of the ground acting on your feet to stop you from falling into the center of the Earth. However, it feels like you are pushing down on the Earth, but that's technically a pseudo-force. Another way of looking at it is to imagine that you are standing in an elevator in space that is accelerating with the speed of 1g. That scenario will feel exactly the same as if that elevator was standing still on the surface of the Earth, but it's more clear that the elevator is pushing you, and not the other way around.