In philosophy, the Rietdijk–Putnam argument, named after C. W. Rietdijk and Hilary Putnam, uses 20th-centuryfindings in physicsspecifically in special relativityto 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 and Putnam. It is sometimes called the Rietdijk–Putnam–Penrose argument.
advanced a form of this argument that has been called the Andromeda paradox in which he points out that two people walking past each other in the street could have very different present moments. If one of the people were walking towards the Andromeda Galaxy, then events in this galaxy might be hours or even days advanced of the events on Andromeda for the person walking in the other direction. If this occurs, it would have dramatic effects on our understanding of time. Penrose highlighted the consequences by discussing a potential invasion of Earth by aliens living in the Andromeda Galaxy. As Penrose put it: The "paradox" consists of two observers who are, from their conscious perspective, in the same place and at the same instant having different sets of events in their "present moment". Notice that neither observer can actually "see" what is happening in Andromeda, because light from Andromeda will take 2.5 million years to reach Earth. The argument is not about what can be "seen"; it is purely about what events different observers consider to occur in the present moment.
Criticisms
The interpretations of relativity used in the Rietdijk–Putnam argument and the Andromeda paradox are not universally accepted. Stein and Savitt note that in relativity the present is a local concept that cannot be extended to global hyperplanes. Furthermore, David Mermin states:
