The Andromeda Paradox comes from Roger Penrose’s interpretation of special relativity and the concept of simultaneity—what counts as “now” for different observers.
A Simple Example
Imagine two people standing on Earth, Alice and Bob. Alice is standing still, while Bob is walking briskly toward the Andromeda Galaxy.
Due to special relativity, motion affects what an observer considers to be “simultaneous” across vast distances.
Since Bob is moving toward Andromeda, his “now” includes a moment in which an alien invasion fleet has already left and is heading toward Earth.
Meanwhile, Alice, who is stationary, considers “now” to be at an earlier point where the aliens haven’t yet decided to launch the invasion.
Why is this weird?
Both are standing right next to each other, in the same place on Earth, yet they disagree about what is “happening right now” in the Andromeda Galaxy—by potentially thousands of years.
It’s a mind-bending consequence of relativity of simultaneity, showing that what we consider “the present” isn’t absolute when dealing with cosmic distances.
Rietdijk–Putnam Argument
In philosophy, the Rietdijk–Putnam argument, named after C. Wim Rietdijk and Hilary Putnam, uses 20th-century findings in physics – specifically in special relativity – to support the philosophical position known as four-dimensionalism.
If special relativity is true, then each observer will have their own plane of simultaneity, which contains a unique set of events that constitutes the observer’s present moment. Observers moving at different relative velocities have different planes of simultaneity, and hence different sets of events that are present. Each observer considers their set of present events to be a three-dimensional universe, but even the slightest movement of the head or offset in distance between observers can cause the three-dimensional universes to have differing content. If each three-dimensional universe exists, then the existence of multiple three-dimensional universes suggests that the universe is four-dimensional. The argument is named after the discussions by Rietdijk (1966) and Putnam (1967). It is sometimes called the Rietdijk–Putnam–Penrose argument.
