The Effect of Gravity on the Universe (1982)

A sealed steel box contains my own weight of "meat bones and blood".  On the blackboard a calculation of the gravitational effect produced by that amount of matter in slowing the recession of the (then) most distant known galaxy.  The calculation is first shown according to Newtonian physics and then with a relativistic correction.  But a small illustration demonstrates the impossibility of an absolute frame of reference.  Mind and matter perhaps.