I saw an interesting question on the Big Question blog tonight:
Q: What is gravity made out of?
A: Gravity is made of mass. No mass. No gravity.
Mass has a natural affinity for mass. The attraction works in much
the same way that tiny bubbles in a liquid (the medium) tend to merge
into larger bubbles — with the difference being that the medium of
gravity is the vacuum that exists in both the atomic centre and in all
intervening distances between elemental masses, which, to an atom,
are indistinguishable from the vacuum of space. The concentric layers
of the atomic structure maintain a state of separation between the
internal and external vacuums creating an interactive ‘anti-medium’
which facilitates and exposes the observed ‘attractive’ mass effect.
In a somewhat wry, McLuhan-esque way: The medium is the mass–age.
For more on gravity…