Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

The comic shows a bedtime scene where a parent (who appears to be a physicist, given the glasses and general SMBC nerd archetype) is reading a story to a child. The parent is saying: "And, thanks to this incredible sensitivity, they were later able to use her to detect gravitational waves." The caption below reads: "Physicists have a different version of 'The Princess and the Pea.'"

In the original fairy tale by Hans Christian Andersen, a princess proves her royal lineage by being so sensitive that she can feel a single pea placed under twenty mattresses. The physicist parent has rewritten the story so that the princess's extraordinary sensitivity is not wasted on merely proving she is royalty -- instead, her ability to detect tiny disturbances is repurposed for scientific use: detecting gravitational waves.

The Humor

The joke plays on the parallel between the princess's legendary sensitivity and the extraordinary precision required to detect gravitational waves, which cause distortions in spacetime on the order of a thousandth of the diameter of a proton. The LIGO detector that first observed gravitational waves in 2015 is one of the most sensitive instruments ever built, so a person who can feel a pea under twenty mattresses would indeed be a useful piece of scientific equipment.

The child''s unamused expression adds to the comedy -- this is clearly not the enchanting bedtime story she was hoping for. The comic is a classic SMBC gag about how scientists cannot help but optimize everything, even fairy tales, for scientific utility.

References

The fairy tale "The Princess and the Pea" was published by Hans Christian Andersen in 1835. Gravitational waves were first directly detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) collaboration in September 2015, confirming a major prediction of Albert Einstein''s 1915 general theory of relativity. The detection required measuring spacetime distortions smaller than one ten-thousandth the diameter of a proton, making it one of the most sensitive measurements in the history of science.