Sean Carroll in Smithsonian Magazine:
When Albert Einstein’s good friend Michele Besso died in 1955, just a few weeks before Einstein’s own death, Einstein wrote a letter to Besso’s family in which he put forward a scientist’s consolation: “This is not important. For us who are convinced physicists, the distinction between past, present, and future is only an illusion, however persistent.”
The idea that time is an illusion is an old one, predating any Times Square ball drop or champagne celebrations. It reaches back to the days of Heraclitus and Parmenides, pre-Socratic thinkers who are staples of introductory philosophy courses. Heraclitus argued that the primary feature of the universe is that it is always changing. Parmenides, foreshadowing Einstein, countered by suggesting that there was no such thing as change. Put into modern language, Parmenides believed the universe is the set of all moments at once. The entire history of the universe simply is.
Today we would call this the “eternalist” or “block universe” view—thinking of space and time together as a single four-dimensional collection of events, rather than a three-dimensional world that evolves over time. Besides Parmenides and Einstein, this picture is shared by the Tralfamadorians, an alien race who appear in Kurt Vonnegut’s novel Slaughterhouse-Five. To a being from Tralfamadore, visiting the past is no harder than walking down the street.
This “timeless” view of the universe goes against our usual thinking. We perceive our lives as unfolding. But it has adherents even in contemporary physics. The laws of nature, as we currently understand them, treat all moments as equally real. No one is picked out as special; the laws simply say how any moment relates to the previous one and to the next.