The Andromeda Paradox Explained
What special relativity shows us about the fluidity of time
Published in
6 min readMay 9, 2020
Bob is sitting on a park bench feeding pigeons. He sees Alice heading in his direction, enjoying a morning run.
“How are you?” he calls.
Breathless, she responds. “Fine. But an armada from the Andromeda Galaxy has just departed for Earth.”
Bob laughs. “No, it hasn’t!”
Who’s right? According to Special Relativity, they could both be right. Or both wrong. It depends.
This is the Andromeda Paradox. It’s a form of the Rietdijk–Putnam argument described by Roger Penrose in his 1989 book, The Emperor’s New Mind. It illustrates one result of special relativity: loss of simultaneity.
Here’s how it works.
