This debate comes up every time I watch a time travel movie: Is the timeline fixed, or is it constantly changing depending on what we do? Turns out that this is also an argument among physicists, some of whom believe the passage of time is illusion — while others view it as very real, and changing all the time.

The scientists aren't debating time travel exactly, though it does come up. Instead, some are arguing that all events in time exist simultaneously, and we humans with our sad little linear, biological lives simply have an illusion that we are moving forward through events that are always already happening. Other physicists believe that time is, in fact, unfolding in real time — which means that we are changing the future all the time. Maybe physical laws themselves are even changing.

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Physicist Sean Carroll explains the basic outline of the debate in a recent column for Smithsonian Magazine:

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 ...

There has, predictably, been some pushback. Tim Maudlin, a philosopher, and Lee Smolin, a physicist, have argued vociferously that time is real, and that the passage of time plays what we might call a generative role: It indeed brings the future into existence. They think of time as an active player rather than a mere bookkeeping device.

Read more at Smithsonian Magazine.