Theory
Explanation
The Joke
A person dismissively tells a scientist, "You experimentalists are nothing! You're just a bunch of stamp collectors!" The scientist, offended, asks, "What do theorists do that's so great?" The theorist proudly responds, "We imagine all the stamps that could exist!" -- and is shown striking a dramatic, grandiose pose while making this declaration.
The comic references a famous (and possibly apocryphal) quote attributed to Ernest Rutherford: "All science is either physics or stamp collecting." This was a dig at fields like biology and chemistry, which Rutherford saw as merely cataloguing and categorizing things rather than discovering fundamental laws. Here, Weinersmith flips the insult by having a theorist use "stamp collecting" as a put-down for experimentalists -- but then revealing that the theorist's own work is essentially the same thing, just hypothetical. Theorists "imagine" stamps rather than collecting real ones, which is arguably even less productive.
The Humor
The humor lies in the theorist's absolute confidence and dramatic flair while describing something that sounds even less impressive than what the experimentalist does. At least the experimentalist interacts with reality -- the theorist just imagines possibilities. The comic satirizes the sometimes-pretentious divide between theoretical and experimental science, where theorists can look down on experimentalists for "merely" doing measurements, while the theorists themselves are doing something that could be characterized as elaborate speculation. The grandiose physical pose of the theorist while making this underwhelming claim amplifies the comedy.
References
The "stamp collecting" insult is widely attributed to physicist Ernest Rutherford, who reportedly said "All science is either physics or stamp collecting" -- dismissing non-physics disciplines as mere taxonomy.