philosophy Philosophy ('love of wisdom' in Ancient Greek) is a systematic study of general and fundamental questions concerning topics like existence, reason, knowledge, Value (ethics and social sciences), value, mind, and language. It is a rational an ...

, the Rietdijk–Putnam argument, named after and

Hilary Putnam Hilary Whitehall Putnam (; July 31, 1926 – March 13, 2016) was an American philosopher, mathematician, computer scientist, and figure in analytic philosophy in the second half of the 20th century. He contributed to the studies of philosophy of ...

, uses 20th-century findings in physicsspecifically in

special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity, "On the Ele ...

to support the philosophical position known as

four-dimensionalism In philosophy, four-dimensionalism (also known as the doctrine of temporal parts) is the ontological position that an object's persistence through time is like its extension through space. Thus, an object that exists in time has temporal parts ...

. If special relativity is applied to all space and time, 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 An argument is a series of sentences, statements, or propositions some of which are called premises and one is the conclusion. The purpose of an argument is to give reasons for one's conclusion via justification, explanation, and/or persu ...

is named after the discussions by Rietdijk (1966) and Putnam (1967). It is sometimes called the Rietdijk–Putnam–Penrose argument.

Andromeda paradox

Roger Penrose Sir Roger Penrose (born 8 August 1931) is an English mathematician, mathematical physicist, Philosophy of science, philosopher of science and Nobel Prize in Physics, Nobel Laureate in Physics. He is Emeritus Rouse Ball Professor of Mathematics i ...

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 on the street could have very different present moments. If one of the people were walking towards the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

, then events in this galaxy might be hours or even days behind the events on Andromeda for the person walking away from the galaxy. 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". Note that neither observer can actually "see" what is happening in Andromeda, because light from Andromeda (and the hypothetical alien fleet) 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. Howard Stein and Steven F. Savitt note that in relativity the ''present'' is a local concept that cannot be extended to global hyperplanes. Furthermore, N. David MerminMermin, N. David (2005
''It’s About Time''
Princeton University Press, Princeton (NJ), 2021, ISBN 978-0-6912-1877-9 states:

Andromeda paradox

Criticisms

References

Further reading