When you're watching Marvel blockbuster Avengers: Endgame, it's OK to sit back and enjoy the ride as our heroes try to save the universe. But if you're one of those people who hopes the MCU movie's key plot points rest on a solid scientific foundation, we have some bad news.
First, though, if you haven't watched the film, STOP READING NOW. This is your spoiler warning.
Because what we're discussing here is time travel, the key way Iron Man and the rest of the Avengers move on from their post-Thanos gloom. The characters take pains to distinguish their time travel approach from Back to the Future and all the rest, but Hollywood, not Heisenberg, is calling the shots.
"Very simply stated, I don't believe that anything that we have learned about gravity and quantum mechanics suggests that time travel to the past is possible," said Leonard Susskind, an influential Stanford professor whose claim to physics fame is helping to create string theory, a fundamental rethink of the universe's laws.
Even the most plausible approach to time travel -- wormholes that could link different points in the universe's fabric of time and space -- requires some seriously weird and potentially nonexistent forms of matter. You're not going to be able to travel back in time to murder your grandfather, much less resurrect the Marvel Cinematic Universe's Spider-Man franchise.
But the good news for time travel fans is that physicists have taken time travel more seriously in recent decades. Let's have a look.
Thank you, Albert Einstein and your crazy math
The main reason for optimism: the mathematics of Einstein's general theory of relativity open up some very interesting possibilities.
"General relativity can permit time travel," famed physicist Stephen Hawking said. "Science fiction fans need not lose heart."
General relativity describes the universe as four dimensions -- the three familiar ones of space, bonded to one of time. It explains gravity as a bending in this spacetime. Relativity is weird and blew a lot of people's minds, but you're probably familiar with the idea visualized in two-dimensional space with suns and planets represented by heavy balls stretching a rubber sheet. Einstein's results are verified with phenomena like the slightly slower rate that GPS satellite clocks tick compared with those on Earth and the phenomenon of gravitational lensing in which massive galaxies bend the path of light traveling from more distant galaxies.
Walking a loop in three-dimensional space isn't mind-blowing. But the math of relativity also opens up another option to travel in a path that includes the time dimension -- what's called a closed timelike curve. In effect, traveling such a curve could return you to your starting point at the same moment you left.
Real research into time travel often hinges on structures that support closed timelike curves. Those include black holes, infinitely long cylinders -- and the most popular time travel idea, wormholes.
Wormholes are a fixture in sci-fi movies for good reason. They offer a way to hook up different places and times in the universe, with two different mouths connected by a tube to traverse from one side to the other.
Well, at least if they exist.
The key to wormhole time travel is to accelerate one of its two mouths to near light speed so a clock there ticks comparatively slowly, then return the mouth back to the unmoved mouth where time has passed more slowly. "By traversing the wormhole from right mouth to left, one can travel backward in time," said Caltech's Nobel Prize-winning physicist Kip Thorne, who helped legitimize scientific investigations of time travel, and colleagues in a 1998 paper on wormholes and time travel.
Here's one problem, though: You'd have to keep the wormhole from collapsing.
"The connection between one part of spacetime and the other part doesn't last long enough for anything to go through," said Richard Muller, a physics professor at the University of California, Berkeley, and author of Now, The Physics of Time. Each end of the wormhole would collapse into a black hole.
With a form of so-called exotic matter -- something with negative energy density -- you could prop wormholes open, but nobody knows if this is even possible, except at the minutest quantum-mechanical scales.
"You need something very special," Muller said. "Even dark energy doesn't do this," he said, referring to the relatively recently discovered phenomenon that's causing the universe's expansion to accelerate.
Another problem: To make a wormhole, you'd have to bend spacetime backward. So yeah, doing something as enormous as what the sun does, only in a profoundly different way.
But hey, humanity has figured out some pretty weird stuff, so maybe someday this sort of technology will be within our grasp.
"You could imagine some advanced civilization making them in the future," Muller said of wormhole engineering. "Go back a couple hundred years and nobody would imagine what we'd be able to do with nuclear power."
Einstein's equations show that the faster you travel, the slower time passes compared with the region where you started accelerating. They also show time travels backward if you exceed the speed of light.
Too bad you can't go that fast. "It would take an infinite amount of power to accelerate past the speed of light," Hawking once said.
Relativity does mean you could, in effect, travel into the future. Accelerate in a spaceship for a few decades so you're traveling near the speed of light, then turn around and decelerate, and you'll have aged just those few decades while centuries passed for people on Earth.
We don't have anything close to a propulsion system that could fuel that sort of spaceship. If we could hang out very close to a rapidly rotating black hole, though, time would pass relatively slowly -- as in the movie Interstellar, where an hour on one planet meant seven years passed on Earth. (Thorne, an advisor to the movie, vetted that idea, by the way.)
Getting spun up about Tipler cylinders
Here's another place you could find a closed timelike curve: a really, really big cylinder spinning really, really fast.
"General relativity suggests that if we construct a sufficiently large rotating cylinder, we create a time machine," concluded physicist Frank Tipler in a 1974 paper. Again, there are practical concerns, like the requirement that the cylinder be inconveniently long -- infinitely so, or perhaps just very long if you're close to the middle -- and made of materials strong enough not to fly apart when spun rapidly.
That work inspired science fiction author Larry Niven to write a short story with the exact same title as Tipler's paper: "Rotating Cylinders and the Possibility of Global Causality Violation." (Though Tipler cylinders are common in discussions of closed timelike curves, note that Tipler's later work linking physics and Christianity alienated scientists who expressed skepticism, to put it politely.)
Goodbye, free will
Hollywood could have another big beef with physics: Time travel strips away any actual control any of us has on our own future, Muller said.
"If wormholes exist and we can go backward in time, it's a consequence in physics that we would be not be able to exercise free will. We would not be able to change our minds," he said. He lays out his case in a paradox he calls the tachyon murder involving a gun that can shoot faster-than-light particles.
You could argue that most of Avengers: Endgame runs along a pretty predictable path. But characters who have no real ability to affect their future? Let's hope that never makes it into the movies.
Originally published April 27.
Update, April 29: Adds further background on wormhole engineering.