A pregnant moment in intellectual history occurs when H.G. Wells’s Time Traveller (“for so it will be convenient to speak of him”) gathers his friends around the drawing room fire to explain that everything they know about time is wrong. This after-dinner conversation marked something of a watershed, more telling than young Wells, who had never even published a book before The Time Machine, imagined just before the turn of the twentieth century.
What is time? Nothing but a fourth dimension, after length, breadth, and thickness. “Through a natural infirmity of the flesh,” the cheerful host explains, “we incline to overlook this fact.” The geometry taught in school needs revision. “Now, it is very remarkable that this is so extensively overlooked…. There is no difference between Time and any of the three dimensions of Space except that our consciousness moves along it.”
Wells didn’t make this up. It was in the air, the kind of thing bruited by students in the debating society of the Royal College of Science. But no one had made the case as persuasively as he did in 1895, by way of trying to gin up a plausible plot device in a piece of fantastic storytelling. Albert Einstein was then just a boy at gymnasium. Not till 1908 did the German mathematician Hermann Minkowski announce his “radical” idea that space and time were a single entity: “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”
So spacetime was born. In spacetime all events are baked together, a four-dimensional continuum. Past and future are no more privileged than left and right or up and down. The time dimension only looks special for the reason Wells mentioned: our consciousness is involved. We have a limited perspective. At any instant we see only a slice of the loaf, a puny three-dimensional cross-section of the whole. For the modern physicist, reality is the whole thing, past and future joined in a single history. The sensation of now is just that, a sensation, and different for everyone. Instead of one master clock, we have clocks in multitudes. And other paraphernalia, too: light cones and world lines and time-like curves and other methods for charting the paths of light and objects through this four-dimensional space. To say that the spacetime view of reality has empowered the physicists of the past century would be an understatement.
Philosophers like it, too. “I conclude that the problem of the reality and the determinateness of future events is now solved,” wrote Hilary Putnam in 1967.
Moreover, it is solved by physics and not by philosophy. We have learned…
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