Time Travel. A History

Not everyone would agree that physicists like to debate the reality of time. Sean Carroll writes, “Perhaps surprisingly, physicists are not overly concerned with adjudicating which particular concepts are ‘real’ or not.” Leave that to philosophers, I think he means. “For concepts like ‘time,’ which are unambiguously part of a useful vocabulary we have for describing the world, talking about ‘reality’ is just a bit of harmless gassing.” The business of physicists is to construct theoretical models and test them against empirical data. The models are effective and powerful but remain artificial. They themselves are a kind of language. Still, physicists do get caught up in debating the nature of reality. How could they not? “The nature of time” was the subject of an international essay contest organized in 2008 by FQXi, an institute devoted to foundational questions of physics and cosmology. One winning essay, chosen from more than a hundred, was Carroll’s own: “What If Time Really Exists?” This was a deliberately contrarian exercise. “There is a venerable strain of intellectual history that proclaims that time does not exist,” he noted. “There is a strong temptation to throw up one’s hands and proclaim the whole thing is an illusion.”

A landmark on that road is an essay published in 1908 by the journal Mind, “The Unreality of Time,” by John McTaggart Ellis McTaggart. He was an English philosopher, by then a fixture at Trinity College, Cambridge.^*9 McTaggart was said (by Norbert Wiener) to have made a cameo appearance in Alice’s Adventures in Wonderland as the Dormouse, “with his pudgy hands, his sleepy air, and his sidelong walk.” He had been arguing for years that our common view of time is an illusion, and now he made his case. “It doubtless seems highly paradoxical to assert that Time is unreal,” he began. But consider…

He contrasts two different ways of talking about “positions in time” (or “events”). We may talk about them relative to the present—the speaker’s present. The death of Queen Anne (his example) is in the past, for us, but at one time it lay in the future and then came round to the present. “Each position is either Past, Present, or Future,” writes McTaggart. This he labels, for later convenience, the A series.

Alternatively, we may talk about the positions in time relative to one another. “Each position is Earlier than some, and Later than some, of the other positions.” The death of Queen Anne is later than the death of the last dinosaur but earlier than the publication of “The Unreality of Time.” This is the B series. The B series is fixed. It is permanent. The order can never change. The A series is changeable: “an event, which is now present, was future and will be past.”

Many people found this A series and B series distinction persuasive, and it lives on robustly in the philosophical literature. By a chain of reasoning McTaggart uses it to prove that time does not exist. The A series is essential to time, because time depends on change, and only the A series allows for change. On the other hand, the A series contradicts its own premises, because the same events possess the properties of pastness and futureness. “Neither time as a whole, nor the A series and B series, really exist” is his apparently inevitable conclusion. (I could say “was” because the paper appeared in 1908. But I can also say “is” because the paper exists in libraries and online and, more abstractly still, in the fast-expanding tapestry of interwoven ideas and facts that we call our culture.)

You may have noticed—and if so, you’re more observant than most of his readers—that McTaggart began by assuming the thing he is trying to prove. He considered all positions in time, all possible events, as if they were already laid out in a sequence, points on a geometer’s line, M, N, O, P, arranged from the point of view of God or the logician. Call this the eternal point of view, or eternalism. The future is just like the past: you can see it in the mind’s eye, neatly diagrammed. Our experience to the contrary is merely a product of mental states: memories, perceptions, and anticipations, which we experience as “pastness,” “presentness,” and “futurity.” An eternalist says that reality is timeless. So time is unreal.

In fact this is a mainstream view of modern physics. I won’t say the mainstream view—in these tempestuous days no one can say for sure what that is. Many of the most respected and established physicists espouse the following:

• The equations of physics contain no evidence for a flow of time.

• The laws of science do not distinguish between the past and the future.

• Therefore—do we have a syllogism?—

• Time is not real.

The observer—physicist or philosopher—stands outside and looks in. The human experience of time is suspended for abstract observation. Past, present, and future are bounded in a nutshell.