Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic explores the idea that someday a longevity or immortality treatment will be discovered, framing it as both a "good thing" and a "bad thing." The good thing is obvious: you get to live forever. The bad thing is that your distant descendants would have to deal with an ancestor from centuries past who holds outdated and embarrassing views. The example shows a medieval-looking man insisting that "the Copernican view is just a theory" -- a reference to modern creationist arguments about evolution.

The final panel delivers the real punchline by flipping the perspective: if immortality is discovered in your lifetime, then you get to be the crazy ancestor. A far-future descendant scolds their "great great great great great great grandpa" for using "ultraglasses" to stare at someone inappropriately, and the grandpa defends himself by saying "That's how we did it in the 21st century." The comic cleverly points out that our own era's norms and behaviors will inevitably seem as backward and offensive to future generations as medieval attitudes seem to us now.