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The “Jules Verne Train” from Back To The Future Part III. This is maybe not what Einstein had in mind when he formulated his relativistic thought experiments, but the science can be evaluated here.R. ZEMECKIS / BACK TO THE FUTURE III
The way we travel through time, at a speed of one second per second, is so boring that we take it for absolute granted. Yet according to Einstein’s theory of relativity, we can not only travel forward through time at different rates by accelerating close to the speed of light, we could potentially travel either forwards or backwards by constructing a bridge through two disconnected locations in spacetime. Time travel, either forwards or backwards, has long been a staple of our imaginations and our stories; who wouldn’t want to explore the unseeable future or go back in time to right a past wrong? But getting those stories to be scientifically accurate is another job entirely. Which movies do the best at that? That’s what Ernio Hernandez wants to know, as he asks:
I’m admittedly a fan of time-travel movies (however they explain it). What movie makes the best case for using this plot device accurately?
Let’s take a look at what makes a good time travel movie, and see how your favorites stack up.
If your goal is scientific accuracy, we have to understand what traveling through time looks like. One of the most revolutionary things that Einstein’s relativity brought us was the notion that space and time aren’t separate, absolute entities, but that they’re inextricably linked. The Universe is made of a four-dimensional fabric known as spacetime, and all objects, particles, and radiation exist within it. This leads to an odd, not-necessarily-intuitive phenomenon: your motion through time is affected by your motion through space, and vice versa.
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Any object existing within this spacetime will immediately notice three things:
- other objects in motion relative to them will have their distances shortened and their clocks time-dilated,
- relative to them, light always moves at the same speed: c, the speed of light in vacuum, and
- their motion through spacetime is determined by the curvature of spacetime, which depends on the matter-and-energy around them in the Universe.
If you’re in a particular, fixed frame of reference (like stationary on the Earth’s surface), then anyone who goes in motion relative to you will move a larger amount through space, meaning they’ll move a smaller amount through time.
This is why the famous twin paradox works the way it does: someone leaving Earth and traveling close to the speed of light will age less than their identical twin who remains on Earth. The one who moves through space at a greater speed will experience a slower motion through time. If we begin considering General Relativity, where we include the effects of gravity, being deep in a strong gravitational field will have a similar effect on you: time will pass at what seems to be a normal rate for you, but far away from your location, everyone else will age much faster. This reaches its extreme near the singularity of a black hole, after you’ve fallen in past the event horizon.
If you create a massive black hole out of matter, alongside another black hole out of negative mass (which we must theoretically assume exists), you can create a wormhole between the two. Separate them by as far as you want, and accelerate one end of the wormhole close to the speed of light. As long as you’re traveling with that accelerated end, you can step through it whenever you want, and arrive at the other end of the wormhole unscathed. The best part? Because you’ve been traveling close to the speed of light, time has passed differently for you. When you step back through the wormhole, it will be as though virtually no time has passed back at home. You could travel for hundreds of years, and then return to your departure point just seconds after you left. In that sense, traveling back in time could really, physically happen.
But some films, even though they don’t talk about or depict the mechanism of time travel in any detail, do admirably succeed in describing how time travel would actually work. Traveling forward is easy: you go close to the speed of light, you return to your departure point, and now you’re far in the future. This is how Planet of the Apes sent a human far into the future on a dystopian Earth, and why Star Wars is so dissatisfying when they engage the hyperdrive. Going fast has real consequences for the passage of time, bringing you into the future no matter what else you do.
The hyperdrive from Star Wars appears to depict an ultra-relativistic motion through space, extremely close to the speed of light. But no one ages differently from normal, an apparent violation of relativity.JEDIMENTAT44 / FLICKR
The idea of traveling back in time has long fascinated humans, such as in Back To The Future’s Delorean DMC-12. After decades of research, we may have hit upon a solution that’s physically possible, but a Delorean might not be required.ED G2S OF WIKIMEDIA COMMONS
The most-visualized black hole of all, as illustrated in the movie Interstellar, shows a predicted event horizon fairly accurately for a very specific class of rotating black holes. Deep within the gravitational well, time passes at a different rate for observers than it does for us far outside of it.INTERSTELLAR / R. HURT / CALTECH
Bill Murray puts down a pitcher of coffee with Andie MacDowell in a scene from the film ‘Groundhog Day’, 1993. (Columbia Pictures)