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Knowing virtually nothing of GR, and only hints of particle theory, this might be something of a naive question. If I've misunderstood somethings, I would gladly like to know why. Perhaps a more straightforward version of my question is this:

What evidence do we have that all the currently known elementary particles do in fact attract each other gravitationally, and in such a way that is related to their masses?

Hopefully this will help to illustrate things: putting aside other forces like EM, obviously some sort of attractive mechanism exists between matter. For reasons that I hope will be clear soon, I'll say that particles want to be with other like particles, and that with more of those particles present in some region of space, the more attractive that region is to other particles belonging to that same class (like a trendy celebrity vacation spot).

If our way of thinking about gravity is correct, then all particles attract all other particles so that the "class" is trivially the set of all elementary particles and there's no need for me to speak of partitioning particles into exclusively "attractive" classes.

But without testing to see that (non-EM/strong) attractions actually occur between all the different types of elementary particles (e.g. bosons, mesons), how would we know that gravitation is actually fundamental, and that our observations of mutual matter attraction haven't misled us into thinking the equivalence principle is true for all elementary particles? The only scale on which gravity has any appreciable effect is the macroscopic scale, where observable matter is dominated by protons and neutrons, since just about anything else decays too fast to collect. If our observations of gravity are dominated by only a fraction of the particle repertoire with particles of virtually the same mass, could we have mistakenly assumed that mass is a universal indication of gravity? Might cosmic rays say something about this?

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  • $\begingroup$ The argument that the gravity applies to light is that 1) the energy of a photon is proportional to its frequency, and 2) the frequency is blue shifted in a gravity well. It is possible to make a similar argument for all particles. Could any any experiments be done along these lines? Have they? $\endgroup$ – mmesser314 Apr 1 '15 at 13:46
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To my knowledge, measuring any kind of gravitational interaction between sub-atomic particles is impossible with current technology. This is due to the fact that gravity is very weak, and the gravitational interaction between particles is negligible even at the high energy scales of the LHC.

Furthermore, we do not have quantum theory of gravity (yet) hence, we cannot identify these interactions from certain characteristics that a quantum theory of gravity will have. So for example, the Weak interaction does not respect right handed and left handed particles equally. Something similar could be true for gravity, but no one knows of such characteristics yet.

But, since we do have some evidence that gravity should have a quantum version that we haven't discovered yet, and since we do see gravity acting on matter in the macroscopic scale, then we can conclude that individual particles must feel gravity on their own.

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    $\begingroup$ See bnl.gov/npss/files/pdf/SnowNPSS1.pdf. In the section on ultracold neutrons, it says "A neutron gas can be bottled (ρ ~100/cc) using total external reflection. Due to gravity the bottle does not need a lid on top." $\endgroup$ – mmesser314 Apr 1 '15 at 13:35
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My knowledge is okay about these things, so I will do my best. Gravity is a property of space-time that is a vector quantity and is accumulative with matter as it increases in mass and density relative to the volume occupied by that matter and energy. It becomes more significant as matter, in a geometrically defined region of space curves that space. The curving of space then basically tells matter how to behave and move. It would almost seem like a class of particles (hadron; sub class baryon, made of 2 up and 1 down quark= proton, flip the numbers and its a neutron) can be the most stable. The lightest particles tend to be most stable, same goes for leptons. In that sense, classes of particles would have something in common and mass can be seen in terms of energy at a fundamental level. All speculative on the last bit because science doesn't have answers yet, or ways to test the answers we do have. Yet alone, outside of relativity, the view of what space-time is, and how gravity is a property of it come to speculation.

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