penthouse
Explanation
The Joke
The comic shows a man at a podium passionately proposing that all rich people be forced to live in skyscraper penthouses, where — according to Einstein's theory of general relativity — they would technically age faster than people at lower elevations. The caption below reads: "My movement for 'relativistic social justice' hasn't yet found its audience."
The proposal is technically grounded in real physics: according to general relativity, clocks run slightly faster at higher altitudes where gravitational time dilation is weaker. Someone living in a penthouse at the top of a skyscraper does indeed age infinitesimally faster than someone at ground level. The comic imagines someone seizing on this real but utterly negligible physical effect as the basis for a social justice campaign — forcing the wealthy to "age faster" as a form of cosmic punishment.
The Humor
The humor comes from the enormous gap between the technically-correct physics and the practical irrelevance of the effect. The time dilation difference between the top and bottom of a skyscraper amounts to a few nanoseconds over a lifetime — a punishment so infinitesimal that no one could ever perceive it. The comic satirizes both overzealous activists who grasp at any argument to punish the wealthy, and the tendency of physics enthusiasts to fixate on technically true but practically meaningless consequences of relativity. The additional irony is that rich people already choose to live in penthouses, so "forcing" them there is hardly a punishment at all.
References
The comic references Einstein's general theory of relativity (1915), specifically the phenomenon of gravitational time dilation. This effect has been experimentally confirmed even at small altitude differences using precise atomic clocks, as demonstrated in experiments by Hafele and Keating (1971) and more recently with optical clocks by NIST researchers.