Relativity
Explanation
The Joke
The comic begins with the premise that once antimatter production becomes cheap and powerful enough, humanity could use Einstein's special relativity to its advantage. Since relativity means that controlling speed lets you control the passage of time (via time dilation), rockets could travel near the speed of light, causing time to pass much more slowly for the travelers relative to those left behind.
The comic then presents "the good news": time will wait for us while other civilizations advance, meaning we could wake up in a future with high technology. But then it reveals "the bad news": if you calculate where every civilization ends up at the same point in their development, they all arrive at "immortal sex-gods" — and the final panel shows confused aliens saying "We're not alone" as they all converge at the same technological endpoint.
The Humor
The comic sets up what sounds like a serious physics discussion about time dilation and interstellar travel, then subverts it with the punchline that every civilization's ultimate technological trajectory leads to the same place: becoming immortal sex-gods. The joke is that time dilation doesn't give humanity any advantage because every other species is on the same path — skip ahead a few thousand years and you just find a universe full of pleasure-seeking immortals who all arrived at the same conclusion independently.
The humor also comes from the contrast between the grandeur of relativistic physics and the banality of the conclusion. All that beautiful mathematics about spacetime, all that engineering ambition about antimatter rockets — and it all leads to the same destination as every other species in the universe.
Broader Context
The comic references real physics: time dilation in special relativity is genuine, and there are theoretical proposals for antimatter-powered rockets. The Fermi Paradox — the question of why we haven't encountered other civilizations — lurks in the background. SMBC's answer here is a comedic one: maybe every civilization eventually converges on the same hedonistic endpoint, making the universe a very crowded but very predictable place.