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> At the same time, there is no sign yet of gravitons particles that transmit gravity and are essential... I never understood how you can explain gravity as 1)

> At the same time, there is no sign yet of gravitons particles that transmit gravity and are essential... I never understood how you can explain gravity as 1) a bending of space-time caused by mass and then 2) pretend it's a force transmited by "gravitons". If 1) is true then it's an effect (phenomena), not a cause (force). If 2) is true than it's a cause (force), not an effect (phenomena).

jpallen 15y ago
Don't worry, neither do physicists. When people talk about 'quantum gravity' they mean some theory which unites curving of space with the idea of gravity being 'quantised' in particles. Only string theory does this consistently (and here the curvature manifests itself as a background of gravitons), but string theory isn't really a serious competitor for physics beyond the standard model.
lmkg 15y ago
Well, it just could be another form of duality, like wave-particle duality. You'd think waves and particles are mutually exclusive too, but nature disagrees. The other thing to keep in mind is that the two characterizations come from two different branches of physics that to date have still not been fully reconciled. (General) relativity is based on a mathematical conception of space-time where gravity can be modeled by mass bending space. Quantum mechanics models all long-distance forces between particles as local interactions with a mediator particle that represents the force. These two theories have trouble getting along together for a number of reasons (look up "quantum foam"), plus we really have no way of telling if these models are a faithful representation of reality, or are just mathematically equivalent to something totally different.
alphaBetaGamma 15y ago
The way physicists understand this is that the curved space-time view of gravity is an approximation that is only valid when you have a lot of gravitons. In that situation you do not see individual gravitons but only the aggregate effect of the ‘sea’ of gravitons you are in. This is similar to e.g. fluid dynamics, which is only valid at scales much larger than the atomic scale. But if you look close enough, you will see effects that are not consistent with viewing a fluid as a continuum of matter (e.g. minute specks of dust moving randomly because they are hit by individual atoms).
Florin_Andrei 15y ago
That's because a true theory of gravity is still missing. On one hand, we have an approximate high-scale view due to relativity, on the other hand quantum theory says there should be some particles called gravitons that are the carriers of this force. Both these views should become approximations of the real theory of gravity, if and when it's created.

jpallen 15y ago

Don't worry, neither do physicists. When people talk about 'quantum gravity' they mean some theory which unites curving of space with the idea of gravity being 'quantised' in particles. Only string theory does this consistently (and here the curvature manifests itself as a background of gravitons), but string theory isn't really a serious competitor for physics beyond the standard model.

lmkg 15y ago

Well, it just could be another form of duality, like wave-particle duality. You'd think waves and particles are mutually exclusive too, but nature disagrees. The other thing to keep in mind is that the two characterizations come from two different branches of physics that to date have still not been fully reconciled. (General) relativity is based on a mathematical conception of space-time where gravity can be modeled by mass bending space. Quantum mechanics models all long-distance forces between particles as local interactions with a mediator particle that represents the force. These two theories have trouble getting along together for a number of reasons (look up "quantum foam"), plus we really have no way of telling if these models are a faithful representation of reality, or are just mathematically equivalent to something totally different.

alphaBetaGamma 15y ago

The way physicists understand this is that the curved space-time view of gravity is an approximation that is only valid when you have a lot of gravitons. In that situation you do not see individual gravitons but only the aggregate effect of the ‘sea’ of gravitons you are in. This is similar to e.g. fluid dynamics, which is only valid at scales much larger than the atomic scale. But if you look close enough, you will see effects that are not consistent with viewing a fluid as a continuum of matter (e.g. minute specks of dust moving randomly because they are hit by individual atoms).

Florin_Andrei 15y ago

That's because a true theory of gravity is still missing. On one hand, we have an approximate high-scale view due to relativity, on the other hand quantum theory says there should be some particles called gravitons that are the carriers of this force. Both these views should become approximations of the real theory of gravity, if and when it's created.