Is the any experimental evidence that gravity exists in the quantum realm? I saw someone speaking about the gravity of an electron when discussing quantum gravity. What is the smallest mass for which we have experimentally observed a gravitational field? Maybe gravity simply doesn't exist in the quantum realm, no?
 A: The GBAR experiment at CERN is aiming to measure the free-fall acceleration of individual anti-hydrogen atoms and compare it with that of normal hydrogen atoms.
A: Gravity at the quantum level is easily tested by the following cheat:
Because gravitational attraction is so weak, we make up for this by collecting together a very, very large number of quantum particles in one spot, and measure the summed force of attraction they produce on a test mass nearby.
This trick works because every single quantum particle in each of the two masses is exerting a gravitational pull on every single particle in the other, and for relatively small masses (like a pair of bowling balls made of lead in your laboratory, for example) those forces simply add.
Note that this trick doesn't work for objects the size of stars or galaxies, but then they wouldn't fit inside your laboratory either.
Once you know how many particles are in the test masses and you measure the force of attraction between them (as Cavendish did long ago with spheres of lead) you then know the size of the attractive force between any two individual particles- without having to build a detector sensitive enough to measure that force between test masses consisting of one particle each.
This same principle is used to detect neutrinos. Because their interactions with matter are so very uncommon, you just build a truly gigantic detector- big enough to furnish a measurable signal- and then staff it with graduate students, sit back in your office, and wait.
