Time travel is a favorite trope of science fiction going back to at least A Connecticut Yankee in King Arthur’s Court and The Time Machine. It took until the mid-40s for someone to come up with the grandfather paradox, which has been pretty well beaten to death in the years since. (How many times has Star Trek alone used it?)
So I started thinking about time and time travel, primarily to see if I could find a theory that would result in new ideas for a time-travel story. While story ideas were not forthcoming, I did come up with a reasonably interesting idea.
Since Einstein and Minkowski, we’ve become used to thinking of time as the fourth dimension. In normal life, we think about locations basically in terms of three numbers: x, y, z, latitude, longitude, and elevation, Fifth and Broadway on the 14th floor, whatever. But if we want to meet someone at Fifth and broadway on the 14th floor, we have to also tell them what time we’re going to meet, say 1:00 PM. Einstein’s general relativity showed that we have to think about time in general as a fourth dimension for everything, not just dates with the brunette you met on the subway, so we always need x,y,z,t.
Now, imagine we could step back from the universe and look at the whole thing, all at once. Then what we think of as our history becomes a path through the whole four-dimensional universe: Fourth and Broadway on the street at 12:54 PM, Fifth and Broadway on the street at 12:56, in the elevator at 12:58, at the new friends office at 1:00 PM. Physicists call this a world line.
Now, you can also imagine that small changes lead to slightly different world lines: the elevator makes a few extra stops and you’re a minute late, or you took a taxi and you’re a few minutes early but you took a different path. Since we’ve stepped back, with Godlike omniscience we see not only everything that is actually on your world line, but every possible world line — so both of those along with all possible other choices are part of the whole picture, along with every other possible arrangement of the pieces: you took the subway, you walked, a taxi brought you down Broadway from uptown (to the sound of honking and shouting, I think Broadway is one way the other direction). In fact, our omniscient view even includes arrangements that aren’t possible, like the one where you simply levitated, or just disappeared one instant and re-appeared the next, teleporting where you wanted to go.
Now, the interesting thing about this is that time as we think about it sort of disappears. It’s an illusion that comes about because we happen to have reached our current place and time by wandering through a particular succession of “previous” instants.
With this model, time travel becomes a lot simpler and the grandfather paradox goes away. If you come up with a time machine, then when you leave your current time and go back to meet grandfather, you appeared at a particular arrangement of the pieces in which some pieces are arranged like your grandfather and some pieces are arranged like you. You then, outraged by being cut out of the will, shoot the SOB and return in your time machine. When you return, you appear in a particular arrangement of pieces where you exist but you’re appearing out of nowhere. If you go back again and stop yourself, then you return to a world in which things are more or less as they were except that your pieces are arranged as someone who has been back in time twice. No grandfather paradox — you just have moved from one arrangement of the pieces to a different one.
Imagine my disappointment when I found out that I hadn’t thought of it first.
In fact, it was thought of more or less by Ernst Mach in the 19th century, and expanded on by Julian Barbour in his book The End of Time: The Next Revolution in Physics. Here’s how Barbour’s theory is described in Wikipedia:
His 1999 book The End of Time advances timeless physics: the controversial view that time, as we perceive it, does not exist as anything other than an illusion, and that a number of problems in physical theory arise from assuming that it does exist. He argues that we have no evidence of the past other than our memory of it, and no evidence of the future other than our belief in it. “Change merely creates an illusion of time, with each individual moment existing in its own right, complete and whole.” He calls these moments “Nows”. It is all an illusion: there is no motion and no change. He argues that the illusion of time is what we interpret through what he calls “time capsules,” which are “any fixed pattern that creates or encodes the appearance of motion, change or history.”
Barbour’s an interesting fellow: he’s quite brilliant, did his Ph.D. at the Univarsität Köln, and then decided he didn’t want to be a physics professor, but just a physicist. He’s supported himself as a translator while doing physics on his own. Here’s a video of him giving a really interesting talk on his theory and the history of the concepts of time.
Now, Barbour’s not exactly considered a crank, but he’s certainly considered a little eccentric, and a lot of people look at his work as being sort of speculative philosophy rather than “real physics”.
Well, maybe not. It’s been known from the start that it’s very hard to make quantum theory and general relativity work together; John Wheeler and Bryce DeWitt developed the Wheeler-DeWitt equation that solves some of the problems, but introduces one of its own: it doesn’t have any room for an idea of time. Don Page and William Wooters then came up with an idea that sort of resurrects at least the illusion through quantum entanglement, the way that quanta that have interacted at any point in the past remain connected even though they’re now separated physically. A cool theory, but one of those things that seemed only theory.
So, along comes a new paper, published on arxiv.org — where all the cool kids in physics hang out — Time from quantum entanglement: an experimental illustration. The authors constructed an experiment in which they set up a very simple artificial system in which they arranged both an internal clock, and an outside observer, and guess what? As predicted by Page and Wooters, to the internal observer, time appears to be passing; to the external observer, the system appears static.
I remember in fourth grade, watching the classroom clock at 2:57, waiting for the bell to ring at 3:00. It seemed sometimes like time had stopped.
I guess maybe I was right.