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- Thread starter metrictensor
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Yes,the two CLASSICAL fields have many things in common,but so many different.

Daniel.

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The two differences that come to mind right away are that gravity is only attractive and that there is no analog to the magnetic field in gravity. But just because these have not been detected does not rule out their existence. We need to keep the options open until they are explicitly rulled out.dextercioby said:

Yes,the two CLASSICAL fields have many things in common,but so many different.

Daniel.

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Theoretically, there is an analog to the magnetic field in gravity. This is sometimes called gravitomagnetism, and other times called frame dragging.metrictensor said:The two differences that come to mind right away are that gravity is only attractive and that there is no analog to the magnetic field in gravity. But just because these have not been detected does not rule out their existence. We need to keep the options open until they are explicitly rulled out.

Some detailed study of the orbits of the Laegos satellites seems to confirm the theoretical predictions of gravitomagnetism/frame dragging, and Gravity Probe B should provide a more direct and precise measurement of this effect.

See for instance

http://www.phys.lsu.edu/mog/mog10/node9.html

for more details.

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selfAdjoint

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This is what Einstein and Schroedinger were working on. http://www.einstein-schrodinger.com/ has carried the work farther and claims to have achieved a full EM field in the combined theory by careful choice of parameters.metrictensor said:

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Chronos

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GR and EM cannot be unified classically.

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Not a very scientific statement.Chronos said:GR and EM cannot be unified classically.

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Certainly yes, but a correct statement.metrictensor said:Not a very scientific statement.

For a more complete explanation, it should be (for example) good to consider the mathematical difficulties encountered by the Theory of Spinors (TS) to connect with the GR. As one knows that the TS is an excellent tool to describe rotations and reversals, that the Faraday's tensor (the EM strength tensor; 4-D representation) accepts a (4-4) matrix representation that can be considered as the representation of a bivector and that this one is equivalent to an infinitesimal rotation... one can may be guess the end of this unsuccess story and understand the statement. Regards

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Chronos said:GR and EM cannot be unified classically.

Yes they can.

Kaluza-Klein, by extending GR to five dimensions you get Maxwell's equations as well.

Unless i am misunderstanding Kaluza-Klein?

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The point is:do you *understand * this theory...??Altogether,not only the results...

Daniel.

Daniel.

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We certainly could have a long discussion as long as we would not have a common definition of the word "classical". If this word is a reference to a certain period of the history, this must be to the period +- 1890- 1920. EM theory was formulated in a 3-D space with consideration for the evolution of the things that is with consideration for the time. The GR is a kind of generalization of the Special Relativity which is introducing the time as an "adult" independant dimension in the discussion and we get with this a 4-D Theory. If we stop the history here, then the Klein-Kaluza Theory which is obviously an interesting generalization of the GR is no more classical. As you see just a question of definition concerning "classical".... Regardsdextercioby said:The point is:do youunderstandthis theory...??Altogether,not only the results...

Daniel.

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dextercioby said:The point is:do youunderstandthis theory...??Altogether,not only the results...

Daniel.

In a word: no. I'll admit that.

But if my understanding of what it means is incorrect, please reeducate me.

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Daniel.

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Blackforest said:We certainly could have a long discussion as long as we would not have a common definition of the word "classical". If this word is a reference to a certain period of the history, this must be to the period +- 1890- 1920. EM theory was formulated in a 3-D space with consideration for the evolution of the things that is with consideration for the time. The GR is a kind of generalization of the Special Relativity which is introducing the time as an "adult" independant dimension in the discussion and we get with this a 4-D Theory. If we stop the history here, then the Klein-Kaluza Theory which is obviously an interesting generalization of the GR is no more classical. As you see just a question of definition concerning "classical".... Regards

Classical in physics generally means non-quantized. Continous quantities and the like.

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Daniel.

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Chronos

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chronos said:GR and EM cannot be unified classically.

Indeed, but Kaluza-Klein does not unify GR and EM any more than Einstein-Maxwell and produces results that do not agree with observation. Einstein spent half his life trying to classically unify GR with EM without success. No modern theorists, AFAIK, seriously consider it possible. Of course you need to define what is 'classical' to lend any meaning to such a statement. I generally think of it as the pre-quantum physics era. Perhaps it would be more rigorous to define it as solutions possible using only the mathematical tools available before the later half of the 20th century.franznietzsche said:Yes they can.

Kaluza-Klein, by extending GR to five dimensions you get Maxwell's equations as well.

Unless i am misunderstanding Kaluza-Klein?

The problem inherent to both GR and EM is they are incomplete. It is understandably difficult to unify theories that are themselves incomplete. Neither GR or EM are quantum theories, hence they are not considered theories of fundamental interactions, as are quantum theories. Maxwell's EM is an effective field theory or "classical" limit of a renormalizable quantum field theory - quantum electrodynamics [QED]. A renomalizable quantum field theory of gravity has not been found and there is no apparent way to unify EM and GR without a quantum theory of gravity. Bear in mind the only sucessful unification theory developed to date - GWS electroweak - came from QED. I seriously doubt any other successful unification theory will be achieved using a 'classical' approach. Perhaps string theory will ultimately triumph. But, you can hardly call string a classical approach with a straight face.

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Ok, this i did not know. I had assumed that KK was simply not pursued because with the discovery of quantum mechanics, maxwell's equations would have been outdated(the 'classical' limit), and so unifying GR with Maxwell would have been moot.Chronos said:Indeed, but Kaluza-Klein does not unify GR and EM any more than Einstein-Maxwell and produces results that do not agree with observation.

I have read about good things coming in the field of loop quantum gravity, which surpisingly, i rarely ever hear mentioned here, while string theory is brought up all the time.A renomalizable quantum field theory of gravity has not been found and there is no apparent way to unify EM and GR without a quantum theory of gravity. Bear in mind the only sucessful unification theory developed to date - GWS electroweak - came from QED. I seriously doubt any other successful unification theory will be achieved using a 'classical' approach. Perhaps string theory will ultimately triumph. But, you can hardly call string a classical approach with a straight face.

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Chronos

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This is a ridiculous statement. How can you say string advocates are more passionate? This is completely ill-logical.Chronos said:

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LQG is very interesting and it seems that some comments here are made w/o knowledge of what LQG is doing. It is not like string theory in being an attempt to unify the 4 fundamental forces. It is only a quantum theory of gravity. The problem with quantum field theories, including QED, is that they are not background independent. As long as a theory assumes an independent background it will not be compatible with GR.franznietzsche said:Ok, this i did not know. I had assumed that KK was simply not pursued because with the discovery of quantum mechanics, maxwell's equations would have been outdated(the 'classical' limit), and so unifying GR with Maxwell would have been moot.

I have read about good things coming in the field of loop quantum gravity, which surpisingly, i rarely ever hear mentioned here, while string theory is brought up all the time.

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selfAdjoint

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See the comments by Lubos Motl on any post at Not Even Wrong that disparage or even criticise string physics.metrictensor said:This is a ridiculous statement. How can you say string advocates are more passionate? This is completely ill-logical.

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jcsd

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Obviously there will be passionate advocates of both sides. The point is that to make such a broad generalization is absurd.selfAdjoint said:See the comments by Lubos Motl on any post at Not Even Wrong that disparage or even criticise string physics.

Here is debate a on the virtues of LQG and ST. Although this is one isolated incident it is obvous who wins and is indicative of the old guard's thinking.

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LQG and string theory may also help one another. There are good qualities of each theory that the other does not have. For example, LQG is background independent while string theory is not. The techniques used by LQG to resolve this may be applicable.jcsd said:

As far as a low energy limit it is possible that LQG could come out of ST in this sense. What is important to understand is that they are not competing unified theories. LQG is simply a quantum theory of gravity. It does not address the standard model in the way ST does.

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