In a research paper with seemingly tragic consequences for science fiction fans, researchers in Hong Kong have confirmed that individual photons can’t move faster than light.
(Hopes had been raised a few years back, when a pair of German physicists claimed to have broken the light speed barrier by propagating waves that effectively arrived before they departed. I said at the time that I could do just as well without all the fancy lab equipment — all I need is a yardstick and an axe. If I chop off the last 12 inches, then the center of the yardstick moves from the 12″ mark to the 18″ mark — a six inch advance in exactly zero time. Of course a sane person might argue that this six inch “advance” involves no actual forward motion, but if I correctly understood the German paper, the same sort of objection would apply there as well.)
It’s being reported that the results from Hong Kong doom all hope for time travel. That’s true or false depending on how restrictively you interpret the phrase “time travel”. Back in 1938, Kurt Goedel (yes, the great logician Kurt Godel — though this particular work has nothing to do with his work in mathematical logic) constructed an example of a universe — that is, a structure that obeys all the laws of physics as laid down by Einstein — that is completely filled with closed timelike curves. If you lived in that universe, you’d be able to travel only forward in time, but still eventually come back to your starting point in both time and space — just as a bug can go in just one direction around a circle and still come back to its starting point. Just like Groundhog Day.
(Well, not just like Groundhog Day, because Bill Murray never returns to the exact same place in space and time — he always returns to a slightly altered version of that place, altered, for example, by the memories he brings along.)
To be more precise: If you lived in Godel’s Universe, you’d always have the option of traveling back into your own past (while always traveling forward in time), but you’d never choose to exercise that option (because while these forward-into-the-past paths exist, no matter ever travels along them). Conceivably there is some variant of Godel’s Universe in which that option does get exercised. Maybe some reader knows more about this.
We (almost surely) don’t live in anything like Godel’s Universe. (Thought question: How would we know?) But the existence of that Universe tells us that the laws of physics can’t rule out Godel-style time travel — and more generally that the laws of physics sometimes allow far more than we expect them to. There are a lot of universes out there, and some of them are pretty weird.
