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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

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.

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.