How Much Mass Required to Demonstrate Gravity?

[Roman Mithman]

If one were to demonstrate gravity independent of earth's constant gravity, how could it be done? Assuming this would need to be done in space, what minimum proportions of mass would be required to demonstrate gravitational pull to a human's naked eye?

 

[Orodruin]

The human eye is a very inaccurate instrument. That’s why this was historically done using a torsion balance: https://en.m.wikipedia.org/wiki/Cavendish_experiment

 

[Ibix]

Two 10kg masses 1m apart in space and initially at rest would collide fairly quickly. Setting $s=\frac 12\frac{GM}{r^2}t^2$ gives an upper bound of about 8 hours (remembering each mass only needs to travel $s=0.5\mathrm{m}$).

Of course, you need to arrange that the masses are initially stationary to better than 0.01mm/s, which is non trivial.

 

[Vanadium 50]

If it needs to be in space, far from the earth, and you need to see it with the unaided eye, it needs to be at least planet-sized. (Note that this is independent of measuring gravity)