Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

A woman tells her companion that she is a "moral relativist." Her companion assumes she means she does not believe in the concept of moral truth -- the standard philosophical meaning of moral relativism. But no: she means she bases her ethical system on her velocity relative to Earth. She explains that the closer she is to lightspeed relative to other humans, the more they appear to be living their lives in an instant due to time dilation. The more fleeting their lives appear, the less significant they seem to her.

She continues this dark logic: "How does one assign personhood to a creature that might live and die in the span of one breath?" She concludes that she does not think you should be allowed to kill someone just because they have a slower reference frame. The final panel shows her companion, alarmed, exclaiming "What quaint Newtonian morality!" as they climb what appears to be a mountain, dismissing her moral concern about not killing people as old-fashioned.

The Humor

The comic is built on a pun: "moral relativist" normally refers to the philosophical position that moral judgments are not universally valid, but the character interprets it literally as morality based on Einstein's theory of relativity. This sets up an elaborate and disturbing ethical framework where the moral value of human life depends on your velocity relative to others. From the perspective of someone traveling near the speed of light, other people's lives would pass in an instant due to relativistic time dilation, making them seem insignificant.

The final punchline adds another layer. When the woman argues that you should not kill people just because they exist in a slower reference frame, her companion dismisses this as "quaint Newtonian morality" -- implying that in their world, the truly enlightened relativistic position is that it IS acceptable to kill people in slower frames. This inverts our expectations: the character who seemed horrifyingly amoral turns out to be the moderate voice of reason, while her companion is even more extreme.

References

The comic references Einstein's special theory of relativity, specifically time dilation -- the phenomenon where time passes more slowly for an object moving at high velocity relative to an observer. At speeds approaching the speed of light, a traveler would experience time passing normally while observing external events appearing to speed up dramatically. The joke also plays on the philosophical concept of moral relativism, the view that moral judgments are not universal but depend on cultural, social, or individual perspectives. The dismissal of "Newtonian morality" parallels how Einstein's physics superseded Newton's classical mechanics.