Quantum mechanics does not fit with gravity (GR) - does this mean one of these two theories is wrong or incomplete?
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2$\begingroup$ Possible duplicates: A list of inconveniences between quantum mechanics and (general) relativity? and links therein. $\endgroup$– Qmechanic ♦Commented Aug 16 at 9:19
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$\begingroup$ "Quantum mechanics does not fit well with gravity": this is false statement promulgated widely. General relativity works perfectly well as a quantum theory at low energy scale. It only fails to be predictive above the Planck scale. The same can be said of the Standard Model, where QCD fails to be predictive below the QCD scale at which the confinement mechanism is still an unsolved mystery. Given the similarities in their respective challenges (the former at high energy scale, the latter at low energy scale ), do you claim that "Quantum mechanics does not fit well with QCD"? Obviously not. $\endgroup$– MadMaxCommented Aug 16 at 20:25
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Quantum mechanics does not fit well with gravity (GR) - does this mean one of these two theories is wrong or incomplete ?
Yes. General relativity is a classical field theory and, as such, is incompatible with quantum mechanics. This is not necessarily an insurmountable difficulty. Electromagnetism is also a classical field theory, but it was turned into a quantum field theory through a process called renormalisation, resulting in quantum electrodynamics. Unfortunately, renormalisation does not work when applied to general relativity, and so at the moment we do not know how to turn GR into a quantum field theory.
There are many ideas on how to develop a theory of quantum gravity - a theory that is consistent both with GR at large scales and with quantum mechanics at small scales. If such a theory included gravity as a natural and inevitable element, rather than an extra force that has to be tacked onto quantum mechanics, then that would be a bonus. However, right now we have no empirical evidence that supports any one candidate for quantum gravity over the others.
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$\begingroup$ Is possible one day, we find some particles in mass that cause gravity and turn out that gravity from curved spacetime is completly wrong? $\endgroup$– 22flowerCommented Aug 16 at 14:19
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$\begingroup$ @22flower Yes, it is possible. Some candidate theories of quantum gravity include particles called gravitons, which play the role that photons play in quantum electrodynamics. In such a theory you could perhaps say that it is gravitons that cause gravity, and the curvature of spacetime is a side-effect of gravitons. $\endgroup$ Commented Aug 16 at 17:24
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$\begingroup$ I mean situation where we find that curvature of spacetime turn out to not exist at all.. For me dont make any sense that gravity (GR) is basically geometry stuff, it is more logic story with gravitons just like photons in electromagentism... $\endgroup$– 22flowerCommented Aug 16 at 17:54
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$\begingroup$ @22flower Some physicists would agree with you. But unless and until we find experimental results that can only be explained by gravitons and not by any alternative - which is very difficult - then we cannot be sure that they exist at all. See en.wikipedia.org/wiki/Graviton#Experimental_observation $\endgroup$ Commented Aug 16 at 17:58
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$\begingroup$ Obviusly we dont know what is origin of gravity, what cause gravity so we end up it this problems...youtube.com/watch?v=1j0Xh9XM34M $\endgroup$– 22flowerCommented Aug 18 at 12:14
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Whether a theory is "right" or "wrong" (or whether it is "complete" or not) isn't a binary choice, but rather a sliding scale. Calling the Earth a "sphere" is, strictly speaking, wrong; it's more like a bumpy oblate spheroid. But it's clearly much closer to the truth than saying the Earth is flat. Similarly Newton's theory of gravity is "wrong" in the sense that general relativity provides a more accurate picture of gravity, but Newtonian gravity is very close to correct for everyday purposes and is vastly better than, say, Aristotle's theory of gravity.
General relativity and quantum field theory both predict correct results for the experiments that we can perform today. But if you extrapolate these theories to regimes that we haven't yet tested, inconsistencies appear. So it is almost certain that eventually a better theory or theories will be developed that will apply more broadly than our current theories. Such future theories will have to reproduce the current predictions of general relativity and quantum mechanics in the areas we can test now.
In other words, the new theory or theories will be an incremental improvement, at least in a practical sense (although they may be quite different conceptually).
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$\begingroup$ @22flower both the "right" and "wrong" theories say that mass is the cause of gravitation. Mass is the cause of spacetime curvature. The measurements that we have used up till now to "prove" this are not going to magically become invalid if a new theory comes along! $\endgroup$ Commented Aug 16 at 11:58
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$\begingroup$ @22flower as I said, any new theory may be conceptually different, but it will have to produce the same results as existing theories, at least to precision of current experiments. So it may involve gravitons or strings or whatever, but the result will be as if spacetime were curved. $\endgroup$ Commented Aug 16 at 14:16
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$\begingroup$ @EricSmith Why we even search for gravitons if we consider curved spacetime theory 100% correct? $\endgroup$– 22flowerCommented Aug 16 at 14:22
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$\begingroup$ @22flower Please re-read my answer. Scientists never consider any theory 100% correct. Theories are closer or farther from the truth. General relativity is the best theory we have at this time. Someday there will probably be a better theory. But at the moment we don't have the experimental evidence to help find that better theory. $\endgroup$ Commented Aug 16 at 14:29
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1$\begingroup$ @22flower Science does not deal in reality as such, it deals in theories (mathematical models) and measurements. Gravitons and curved spacetime are parts of various mathematical models, not reality! $\endgroup$ Commented Aug 16 at 14:30