Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

A physicist is at a chalkboard filled with complex equations, and announces the conclusion: that one could go to a pay-by-weight salad bar and earn money by eating cheese, which is "clearly impossible." The caption at the bottom reads: "Disproving the idea of negative mass was remarkably easy."

The comic imagines that the way to disprove the theoretical concept of negative mass in physics is not through sophisticated experiments or deep theoretical analysis, but through a simple reductio ad absurdum argument involving a salad bar. If negative mass existed, a substance with negative mass would weigh less than nothing, meaning at a pay-by-weight salad bar, adding it to your plate would reduce the total weight and thus the price -- eventually making the restaurant owe you money.

The Humor

The comedy works on multiple levels. First, there is the absurd contrast between the incredibly complex-looking physics equations on the chalkboard and the utterly mundane, everyday conclusion about salad bars. Theoretical physics papers typically arrive at profound conclusions about the nature of the universe, not about exploiting restaurant pricing models. Second, it pokes fun at how physicists actually do use reductio ad absurdum arguments -- showing that a hypothesis leads to an impossible or absurd result -- but usually the "absurd result" is something like violating causality, not free cheese. The joke also plays on the real physics concept of negative mass, which has been seriously discussed in theoretical physics contexts.

References

Negative mass is a hypothetical concept in physics where matter would have a negative value of mass. It has been discussed in the context of exotic matter, wormholes, and the Alcubierre warp drive. In 2017, researchers at Washington State University created conditions exhibiting behavior consistent with negative mass using rubidium atoms cooled to near absolute zero.