document: I also could not resist mentioning another wild speculation [10]. Many years ago, inspired by the almost exact correspondence between Einstein's post-Newtonian equations of gravity and Maxwell's equations of motion I proposed the gravitipole in analogy with Dirac's magnetic monopole. After Dirac there was considerable debate on how a field theory of magnetic monopoles may be formulated. Eventually, 't Hooft and Polyakov showed that the magnetic monopole exists as an extended solution in certain non-abelian gauge theories. Most theorists now believe that electromagnetism is merely a piece of a grand unified theory and that magnetic monopoles exist. Might it not turn out that Einstein's theory is but a piece of a bigger theory and that gravitipoles exist? In grand unified theory the electromagnetic field is a component of a multiplet. Could it be that the gravitational field also somehow carries an internal index and that the field we observe is just a component of a multiplet? Throwing caution to the wind, I also asked in [10] if the gravitipole and the graviton might not form a representation under some dual group just as the magnetic monopole and the photon form a triplet under the dual group of Montonen and Olive [11].
extractive_keyphrases: electromagnetic field electromagnetism field theory of magnetic monopoles gravitational field gravitipole gravitipoles graviton magnetic monopole magnetic monopoles photon
abstractive_keyphrases: dirac's magnetic monopole einstein's post-newtonian equations of gravity maxwell's equations of motion non-abelian gauge theories