system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the quantization of gravity so hard to prove? If mass if quantized, why does it not follow that gravity is also quantized?
gpt
>If mass if quantized, why does it not follow that gravity is also quantized?

Mass isn't quantized.  There is no reason, a priori, why a particle cannot have any value for its mass.  Indeed, in the Standard Model, the masses of the fermions are parameters that must be fixed according to experiment.

Also, energy is not quantized either; this is a frequent misconception.  Electromagnetic waves are quantized into packets of energy, proportional to Planck's constant and the wavelength of the EM wave.  Planck's constant is constant, but an EM wave can have *any* wavelength.  To say that it is quantized doesn't necessarily mean that only certain amounts of energy are allowed (although for some systems, like atoms, that may be true); rather, it means that EM waves interact with matter via discrete packets of (any particular value of) energy -- which we call photons.

>Why is the quantization of gravity so hard to prove?

For two main reasons:  (1) gravity is such a weak force that we can't even directly detect coherent gravitational waves (which, if gravity is quantized, would be made up of many, many gravitons); so far we only have indirect evidence of gravitational waves.  That makes it very hard to "prove" (edit:  i.e. demonstrate to arbitrary level of accuracy) things about gravity, since we can't yet measure those waves directly.  Secondly, (2) our current technique for successfully quantizing the other forces doesn't quite work for gravity.  If gravity is quantized, its quantization must be described through some other mathematical framework besides renormalization -- one which we haven't figured out yet.

Also FYI, it's worth mentioning that the recent BICEP2 observation of primordial B-mode polarization in the CMB, if it is independently confirmed, *would* constitute ~~proof~~ demonstration to a high level of accuracy that gravity is quantized in nature.  Though recently there has been some concern that the BICEP2 result might be explainable via polarization due to dust clouds in space.  It remains to be seen whether the result will hold up to peer review, or be confirmed by another experiment.

Hope that helps!