The quest of the elusive graviton.
If found it could unite GR and QM.
Altogether, there are some “fundamental” differences between the way in which we mathematically calculate and physically approach GR versus QM. General relativity being a geometrical framework with space in three dimensions merged with time hence manifesting the famous spacetime concept. Where quantum mechanics is algebra on a very high level. This is the first problem in uniting GR and QM.
The second discrepancy is that the theorized graviton, the expected particle incorporated in modern gravity criteria and hence in GR, obviously goes under the QM “radar”. I.e. having far less energy related to any other known QM particle. For example, the gamma ray photon is much greater than the electron neutrino in energy amounts. -Though the photon is massless when the neutrino is regarded as having a very small mass. And, for the writing moment, the graviton is regarded as having a far minor or extremely small energy amount, combined with being massless; both, versus the photon and the electron neutrino as being among the two weakest of the known elementary particles in any average physics situation.
So, the second problem in merging GR and QM seems to be the graviton’s proposed and theorized dimensions, extremely small energy and zero mass, towards all the other known elementary particles. -Simply, the graviton is very much eluding from all the other elementary particles and goes much, or even extremely, below the present QM “physics possible measurement level” in any attempt to a uniting and coherent universal physics description. -Therefore, this quest of the elusive graviton.