[Physics] the smallest item for which gravity has been recorded or observed

Question

What is the smallest item for which gravity has been recorded or observed? By this, I mean the smallest object whose gravitational effect upon another object has been detected. (Many thanks to Daniel Griscom for that excellent verbiage.)

In other words, we have plenty of evidence that the planet Earth exhibits gravitational force due to its mass. We also have theories that state that all mass, regardless of size, results in gravitational force.

What is the smallest mass for which its gravity has been recorded or observed?

(By the way, I hoped this Physics SE question would contain the answer, but it wound up being about gravity at the center of planet Earth.)

Answer 1

Gravitation effect on neutrons have been demonstrated.

Bouncing neutrons

To obtain neutrons with quantized gravitational energy states, the team used a technique first described in 2011, in which a nuclear reactor produces neutrons travelling at 2,200 metres per second. These are then slowed to less than 7 metres per second and cooled to just a fraction of a degree above absolute zero, before being funnelled between two horizontal plates.

The neutrons bounce off the lower plate, which is a highly polished mirror, while the upper plate is an absorber that creams off those with the highest energies, to leave only neutrons in their lowest quantum state. Neutrons are ideal for these quantum bouncing experiments because they have only weak electrostatic polarization and carry no net electric charge, says study co-author Peter Geltenbort, a physicist at the Laue-Langevin Institute in Grenoble, France, which produced the neutrons for the experiments. “They only really feel gravity,” he says.

I offer the neutron as the smallest particle to display Newtonian gravity.

The team found that the neutrons' energy levels are exactly as if they are being acted on by gravity alone — measured on a scale 100,000 times smaller than ever tested before. This puts limits on additional 'exotic' forces that some have predicted could be seen on these tiny scales.

In this experiment the two objects are "a neutron" and the "earth", obeying Newton's third law of motion:

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.