Time Travel. A History

Notice, by the way, how easy it had already become for physicists and mathematicians to speak of alternative universes. “In these worlds…,” Gödel writes. The title of his paper, when he published it in Reviews of Modern Physics, was “Solutions of Einstein’s Field Equations of Gravitation,” and a “solution” is nothing less than a possible universe. “All cosmological solutions with non-vanishing density of matter,” he writes, meaning all possible universes that aren’t empty. “In this paper I am proposing a solution” = Here’s a possible universe for you. But does this possible universe actually exist? Is it the one we’re living in?

Gödel liked to think so. Freeman Dyson, then a young physicist at the Institute, told me many years later that Gödel would ask him, “Have they proved my theory yet?” There are physicists today who will tell you that if a universe has been proved not to contradict the laws of physics, then yes, it is real. A priori. Time travel is possible.

That’s setting the bar fairly low. Einstein was more cautious. Yes, he acknowledged, “such cosmological solutions of the gravitation equations…have been found by Mr. Gödel.” But he added mildly, “It will be interesting to weigh whether these are not to be excluded on physical grounds.” In other words, don’t follow the math out the window.^*4 Einstein’s caution did little to diminish the popularity of Gödel’s closed timelike curves among fans of time travel—and in their number we must count logicians, philosophers, and physicists. They wasted little time in launching the hypothetical Gödel rocket ships.

“Suppose our Gödelian spacetime traveller decides to visit his own past and talk to his younger self,” wrote Larry Dwyer in 1973. He specifies:

at t₁, T talks to his younger self

at t₂, T enters his rocket to begin his journey to the past.

Let t₁ = 1950; t₂ = 1974

Not the most original start, but Dwyer is a philosopher writing in Philosophical Studies: An International Journal for Philosophy in the Analytic Tradition, a far cry from Astounding Stories. Dwyer has done his homework:

Science fiction contains an abundance of stories where the plot centres around certain individuals who, having operated complex mechanical devices, find themselves transported back to the past.

Besides reading the stories, he is reading the philosophical literature, beginning with Hospers’s proof of the impossibility of time travel. He thinks Hospers is just confused. Reichenbach is confused, too (that would be Hans Reichenbach, author of The Direction of Time), and so is Čapek (Milič Čapek, “Time in Relativity Theory: Arguments for a Philosophy of Becoming”). Reichenbach argued for the possibility of “self encounters”—the “younger ego” meets the “older ego,” for whom “the same occurrence takes place a second time,” and though this may appear paradoxical it is not illogical. Dwyer begs to differ: “It is this sort of talk that has given rise to so much confusion in the literature.” Čapek is drawing diagrams with “impossible” Gödelian world lines. Likewise Swinburne, Whitrow, Stein, Gorovitz (“Gorovitz’s problems, of course, are all of his own making”) and indeed Gödel himself, who misconstrues his own theory.

They all make the same error, according to Dwyer. They imagine that a time traveler could change the past. That cannot happen. Dwyer can live with other difficulties created by time travel: backward causation (effects preceding their causes) and entity multiplication (time travelers and time machines crossing paths with their doubles). But not this. “Whatever else time travel may entail,” he says, “it does not involve changing the past.” Consider old T, using his Gödelian spacetime loop to travel back from 1974 to 1950, when he meets young T.

The encounter is of course recorded twice in the mental history of the time traveller; while young T’s reaction to his encounter with T may be one of fear, scepticism, joy, etc., T, for his part, may or may not recall his feelings when, in his youth, he was confronted by a person claiming to be his older self. Now of course it would be self contradictory to say that T does something to young T which, by his memory, he knows does not happen to him.

Of course.

Why can’t T go back and kill his grandfather? Because he did not. It’s that simple.

—

EXCEPT—of course—it’s never that simple.