Parties
Explanation
The Joke
The comic contrasts how different academic disciplines behave at parties. The physicist talks about how everything is physics. The philosopher questions whether the party even exists. The economist tries to optimize the snack distribution. The mathematician is in the corner proving that a good time is theoretically possible.
The Humor
This is a classic "disciplinary stereotypes" joke, but Weinersmith's version is sharper than most because each stereotype captures something genuinely true about how people in these fields think. Physicists really do tend toward reductionism ("it's all physics at the bottom"). Philosophers really do question premises that everyone else takes for granted. Economists really do think in terms of optimization and allocation.
The joke isn't that these people are weird — it's that their professional training has warped their cognition in predictable and slightly tragic ways. They can't turn it off, even at a party.
Context
SMBC has a large following in academic circles, and "how different fields see the world" is one of its most reliable comedy formats. The humor is affectionate rather than mean — Weinersmith is clearly someone who has spent a lot of time around academics and finds their quirks endearing.